Depends on how wet the paste is. Not really a helpful answer, I know, but it's the truth. I had some paste that was supposed to be 2g (didn't weigh it), after it dried it weighed 1.3g. That's a 35% reduction if the wet weight was correct. However, if you're getting paste that is cut with water it could be extremely wet and you could lose double that.
Any chance you could buy a little first (one, maybe half a gram) and see how much it loses when dried? That should give you a good idea of how much you're going to lose on that specific batch of paste.
Also I'm quite curious how that dealer gets 87% purity. IIRC, pure amphetamine-sulphate (which is the salt you usually get) contains 72% amphetamine and 28% sulphate (by weight). So unless you're getting something else, you're being fed BS. And a dealer who lies about purity probably also wouldn't mind cutting his product with water, or something active.