This puts you in the perfect bargaining position - you don't need to do a deal (indeed don't want to) - they do (probably must).
Make it clear you use it for business purposes and would incur substantial costs to change to a new domain, and then give them a figure that would make it worth your time.
Don't know what that figure would be for you, but I had a similar offer a few years back for a domain I was using just for limited business email and we settled on an equivalent of around $10,000. (it was a .com that a UK company wanted to go with their .co.uk). Nowadays I'd probably ask for more.
So, don't wait for them to inch their price upwards. Just work out how much you'd like and tell them that is the firm price, with a 7 day time limit for acceptance (so they don't keep you hanging on). And if you never hear from them again, you don't care anyway, so there's nothing to lose.