How are you actually predicting AI costs before they hit your invoice?

Switched from prototype to production last month and our AI bill was 3x what we estimated. Not because we picked the wrong model - we just didn't know what we didn't know. Turns out token price cards are the tip of the iceberg. Reasoning models bill internal chain-of-thought tokens at full output rate. Multimodal calls charge per image tile before even reading your prompt. Function calling quietly adds hundreds of system tokens per request. Realtime audio is priced in a completely different unit than text on the same model. And that's just LLMs. Image gen has no standard billing unit across providers. STT providers round audio duration differently and it matters at scale. Agentic loops that trigger web search can quietly add thousands of API calls nobody budgeted for. Genuinely curious how others are handling this. Are you estimating upfront or just reacting to the invoice? And what's the one cost variable that caught you most off guard?