For those who bootstrapped AI startup, how much did development cost? I will not promote

For those who has bootstrapped any AI startup, from AI agents to simple “ChatGPT wrappers”, how much did it cost to develop (exclude developer cost) Me and a tech co founder are planning a side project but wondering how the cost model will work out since AI is heavily usage based, unlike a typical SaaS where hosting cost is negligible during MVP phase. Obviously you will need to perform testing, researching prompts and models, etc. although small, these are variable costs so I wonder how everyone’s experience is like.