What you are describing is more of a closing technique by the recruiter. They are basically trying to lock down your yes before going to work on the manager.

For that reason I would feel totally fine not giving a number, 'I'll cross that bridge when I come to it har har har' or something equally vague.

I'd be wary of over explaining yourself in these situations. A recruiter knows exactly the situation you are faced with and everything they do is designed to turn that in their favor. The reason they ask for how much you would accept on the spot is so you feel obligated to accept on the spot and not go get that 20% more offer because you feel like you already committed.

One tactic that can be used is something like 'I am interviewing with multiple companies and plan on making a decision on X date.' It makes things clear and there is really no arguing with it. This depends on if you actually are lining up your final interviews for a <2 week period. It helps shut down any recruiter closing attempts as you can just repeat that you will make a decision on Friday or whatever like you told them before.