With the GPT-4-Turbo AI model costing $0.01 per 1k tokens in, and $0.03 per 1k tokens out, sending 100 tokens is 1/10 of a cent. Then you pay for how much the AI writes.
However, that input can grow vastly if you are maintaining a long chat history of past interactions, are processing multiple documents, hundreds of lines of computer code, or placing other information into context, up to $1.25 per API call at the max of the longest context length gpt-4-turbo model (or the very limited release gpt-4-32k: $2.88 for 16k in 16k out). It depends on your use, as you pay per data.