Inquiry about gpt 3.5 turbo price

Hello! I have a question about GPT API price.
I will develop an chat program using GPT API.
This chat include user’s questions and GPT’s responds;
I shown the price of GPT-3.5-turbo was $0.002 / 1K tokens on the OpenAI homepage.
But, I don’t know an exact price.
If i use chat program using GPT API,Does tokens include user’s questions and GPT’s responds?
or Does tokens include user’s questions only?

And second question is about Payment method.

If i use chat program using GPT API,When should I pay and How can i pay?
I want to pay all at once.
I expect that develope period using GPT API is 3 months.
After that, I will not use API.