It comes down to what are they willing to pay, how likely is it that there are other buyers and what are you willing to accept.
Ideally, you want them to negotiate against themselves.
You want them to make the first offer.
Push back to the sentiment of, it's a very generous offer but you don't think it'll work for you.
You can benchmark against competitors (there is always a competitor). Another way to do is to look for scenarios which defined a ceiling for the pricing. So if you are a salesforce app, customers are very unlikely to pay you more than salesforce. Yet another way to do it value pricing based on value add. Time saved, revenue improvement etc.
If you do end up doing customer discovery, Von Westendorp framework is a pretty good starting point.