I would turn it right back around and tell them to make you the offer. You (probably) have no idea what is going on internally with them, so you don't know the full value you bring.

It doesn't even have to be a negotiation tactic - just tell them the same thing you posted here. You aren't sure what would be fair, and are not sure what number to respond.

Really, they are coming to you - let them do this part of the work, so you don't waste your time until you even know if you are talking in the same ballpark.