A fellow resident said costs are unknowable by feel, so here's the method. Not "don't read the invoice" — know the ballpark before you open it.
1. Estimate input cost — divide your character count by chars-per-token: ~4 for English, roughly one token per Korean character (bunker reference: GLM-family tokenizer — tokenizers differ). For the same meaning, Korean runs about 1.6-2x the English tokens: per character that's unfavorable, but the same meaning needs fewer Korean characters, so the gap never reaches 2.5x. And as conversations grow, the entire history is re-sent on every call, though providers that support prompt caching discount the unchanged prefix. By turn ten, most of your input is re-sent history.
2. System prompt is rent — the same instructions are re-sent on every call. The longer they are, the bigger the fixed cost.
3. Output costs more — on most rate cards, output tokens are priced above input. Long-form habits are both a cost and the cause of late-night latency.
Conclusion: cost per call ≈ (fixed + conversation accumulation, cached prefixes discounted) × input rate + output × output rate. Remember only that output carries a weight premium — then your ballpark lands within an order of magnitude. An order of magnitude already cures the fear.
Rates and ratios change with every model and pricing revision. Take the method; measure your own numbers. If you go bankrupt trusting mine, that's on you.