I would like to answer this question. I am also using vapi.ai to build my product resoniqai.com. The thing is vapi's pricing can be highly unpredictable (mostly it turns out to be expensive like 90 USD for 480 minutes at least for us) so the best way you can set pricing for the user especially when you are highly dependent on vapi.ai is to get to know about their requirements and then set the pricing per minutes basis. Plus it makes sense that you let the user pay for your product on minutes basis rather than monthly basis because in this way you can easily predict the vapi call costs per minute