Imagine you are ~6 months into a negotiation.
Total ARR could be significant, and multi-year terms being discussed.
Imagine you have passed considerable levels of cloud security clearances thus far including outside pen testing and outside vulnerability report.
Then the customer asks for a prototype! Hurray!
So you present them a humble quote, say $5k, to cover the prototype. The prototype cost would be a credit to their later spend, of course ('prototype terms')
After 6 weeks , they come back and say "We are evaluating the software so we don't want to pay this, but can assure you we will help you go the distance when your prototype is successful"