Why isn't depreciated value fair for an insurance payout? I don't know if the $750 is a low-ball value for the depreciated $3000 bike, but if it is accurate then $750 will get OP a bike of about the same value as the one he lost.
I've had insurance companies low-ball me though, and it's always been a struggle to get to a fair settlement in those cases. With or without a lawyer, who will take a third of it. I think that you have to be prepared to go through with it if you have to, the whole nine yards of civil court, and that means you have to be right in the first place and have it documented. They've heard it all before and won't be bluffed, and might drag you through a protracted fight just to test your resolve.
Personally if the $750 is objectively close I'd take it, or give them a reasonable counter offer.