Price per token has fallen sharply through better hardware, smaller distilled models and serving optimisations. Consumption has grown faster: longer contexts, reasoning models that generate far more tokens per answer, and agents that turn one user action into dozens of model calls. Falling unit prices with rising unit counts produce larger bills.
Almost every AI API bills per million tokens, with separate prices for input and output. Output usually costs several times more than input. Cached input, batch processing and smaller models can each cut the bill substantially, and the total for a conversation grows with history because most APIs re-send the whole thread every turn.