# Big Purchase Decision > Evaluates a major purchase with total cost of ownership, opportunity cost, affordability red lines, and a cooling-off rule, producing a scored buy/wait/walk verdict. Use when someone asks "should I buy this car", "can I afford this", "is this purchase a good idea", or is about to finance anything with a monthly payment. Do NOT use for fitting an approved purchase into the monthly plan - use budget-builder instead; for whether the purchase should outrank debt payoff or savings - use financial-planner; for home-purchase retirement trade-offs - use retirement-projection. - Skill: `skillmedev/big-purchase-decision` (Agent Skill) - Install (CLI): `npx skillmds@latest add skillmedev/big-purchase-decision` - Raw SKILL.md: https://api.skillmd.com/api/skills/skillmedev/big-purchase-decision/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Finance & Business - Author: SkillMedev (https://skillmd.com/u/skillmedev) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/skillmedev/big-purchase-decision --- # Big Purchase Decision Big purchases feel like one-time decisions but are almost always multi-year financial commitments. The costly mistake this skill prevents is deciding on sticker price and monthly payment - the two numbers sellers control - instead of total cost, opportunity cost, and affordability. Analyze before signing, because afterward the analysis is just regret. ## Operating procedure ### Step 1: Confirm the purchase clears the threshold Apply the full framework to any single purchase above **1% of annual take-home income** (for most households, $500-$1,500). Two lighter rules cover everything below it: - Below the 1% threshold but still discretionary and unplanned: apply the **24-hour rule** - sleep on it once. Most impulse purchases do not survive one night. - A useful ceiling for guilt-free discretionary buys: anything under **1% of net worth** rarely warrants analysis for households with positive net worth; above that, it does. Non-urgent purchases above the 1%-of-income threshold get the **30-day rule**: wait 30 days before buying. Most impulse-driven desires fade within that window; a desire still strong at day 30 that also passes the financial tests below is a considered want, not an impulse. ### Step 2: Gather inputs 1. Purchase price and how it would be paid (cash, financing terms, APR). 2. Annual take-home income and current monthly surplus (from budget-builder if one exists). 3. Ownership costs: insurance change, maintenance, fuel/consumables, registration or subscription fees, expected useful life. Label estimates as estimates. 4. Emergency fund status and any debt above 7-8% APR - both are gate conditions in Step 5. ### Step 3: Compute total cost of ownership TCO = price + financing interest + (annual insurance delta + maintenance + consumables + fees) × years of useful life. Divide by useful life for **true annual cost**. A $25,000 vehicle typically lands at **$8,000-$12,000 per year** once interest, insurance, fuel, maintenance, and depreciation are loaded - three to four times what the sticker suggests. ### Step 4: Compute opportunity cost At a 7% average annual real return, $10,000 invested today is worth approximately **$19,700 in 10 years** and **$38,700 in 20 years**. Scale linearly for the purchase amount and state it plainly: "this $30,000 purchase is ~$59,000 of retirement money in 10 years." This is not a reason to never spend - it is the honest price tag for the trade-off. ### Step 5: Apply the affordability red lines Any single "no" here is a **walk or wait**, regardless of desire: - Buying it would drop the emergency fund below the starter month (emergency-fund-planner). - The user carries debt above 7-8% APR and the purchase is discretionary. - Financed: total monthly debt payments including the new one would push debt-to-income above **36%** of gross income (above 43% is the hard red line lenders use). - The monthly ownership cost (TCO ÷ 12 per year of life) exceeds the current monthly surplus. ### Step 6: Evaluate the financing Cash is preferable for discretionary purchases. If financing, the APR is the decision number, not the monthly payment - a lower payment achieved by stretching the term means paying more in total; compute total interest over the full term and add it to TCO. A payment made "affordable" only by a 72- or 84-month term is a red-line signal in itself. ### Step 7: Time and negotiate Where timing is flexible: end of month, holiday weekends for appliances and vehicles, and end of model-year cycles consistently price better. Research fair-market value before negotiating - it is the single most effective way to avoid overpaying. ### Step 8: Fill the scorecard and give the verdict ## Purchase decision scorecard Copy and fill. Verdict rule: any red line failed = WALK/WAIT; all red lines passed and score ≥ 4 of 5 = BUY; 3 = WAIT 30 days and re-score. ``` PURCHASE SCORECARD - [FILL: item] - [FILL: date] Price: $[FILL] Payment method: [FILL: cash / financed at [FILL]% APR, [FILL] months] TCO: $[FILL] over [FILL] years = $[FILL]/year ($[FILL]/month) Opportunity cost: $[FILL] ≈ $[FILL] in 10 years at 7% real RED LINES (any NO = walk/wait) Emergency fund stays above starter month: YES / NO No high-interest (>7-8% APR) debt outstanding: YES / NO DTI with new payment stays under 36%: YES / NO Monthly TCO fits current surplus: YES / NO SCORE (1 point each) [ ] Survived the 30-day rule [ ] Paying cash, or APR under [FILL: best available rate] [ ] TCO researched from real quotes, not guesses [ ] Fair-market value researched; negotiation planned [ ] Still needed if income dropped 20% next year VERDICT: BUY / WAIT 30 DAYS / WALK - because [FILL: one sentence] ``` ## Worked example Used SUV, $25,000, financed $20,000 at 8.5% for 60 months. TCO: $25,000 + $4,600 interest + ($1,400 insurance delta + $1,200 maintenance + $2,200 fuel + $300 fees) × 6 years = $50,200 → **$8,370/year, ~$700/month** - inside the $8,000-$12,000 band. Opportunity cost of the $5,000 down payment: ~$9,850 in 10 years. Red lines: emergency fund holds (YES), no card debt (YES), DTI rises to 31% (YES), $700/month vs $900 surplus (YES). Score: 30-day rule passed, APR beats the local credit-union alternative - no; quotes real - yes; FMV researched - yes; survives income drop - yes. Score 4/5, all red lines pass: **BUY**, negotiating at month-end against the researched FMV. ## Deliverable Produce a completed scorecard: TCO with the per-year and per-month figures, the 10-year opportunity cost, all four red lines answered, the 5-point score, and a one-sentence verdict the user could read back in a year without wincing. ## Do NOT - Do not evaluate on sticker price or monthly payment; both hide the real cost. - Do not let a strong "want" override a failed red line - the red lines exist for exactly that moment. - Do not extend the loan term to make the payment fit; compute what the extension costs in total interest. - Do not skip the 30-day rule because a sale ends soon; recurring sales are a sales tactic, not a deadline. - Do not use invented ownership costs; get an insurance quote and typical maintenance figures, or label the TCO provisional. ## Quality bar - TCO includes financing interest and at least four ownership cost categories, each sourced or flagged as an estimate. - The opportunity cost is stated in dollars at 10 years, not as an abstract principle. - All four red lines have explicit YES/NO answers. - The verdict follows mechanically from the red lines and score - no override without a written reason. ## Escalation This is general financial education, not individualized financial advice. For real estate, business equipment, or complex financing, a financial advisor or accountant should model the full impact including tax implications (route tax questions to tax-optimization); route the question of whether this purchase belongs in the plan at all to financial-planner.