Pretty sure you wouldn't get an answer so why should you feel bad for not answering?
I hate these back-and-forths too where both sides know the game being played but have to be coy about it for no good reason. I'd prefer to just say you're not prepared to share that information and if they pushed be upfront that everyone knows in salary negotiation you shouldn't share this and it's irrelevant if you happened to be underpaid before.