This situation happens all the time, Was it from a fellow 'domainer' looking to pick-up a bargain or possibly a potential end user with a price expectation way below your asking price. We've all had the "But it only cost you $10, so that's all I'm offering" emails. (some think the initial registration is forever)
Don't stress over those that don't reply to your sales price response, consider them dead-wood. Chasing them will just make you look desperate for a sale. They know where to find you if they were serious about negotiating a price.
don't raise your expectations on any email requesting a price, that is unless you've nailed down who the email is from and it's either a top-player or a sizable business/company, then its worth staying on the case