The usual benchmark is the "usual and customary" charges for a procedure. You can look it up for a procedure for your area. You then go to the hospital and point out these charges. My guess is they're much more likely to agree with this than the Medicare rates.
It's also the rate your insurance will use if you go out of network. So if your insurance pays 40% out of network, and you get billed $1000 for a $100 procedure, your insurance will pay only $40 (4%).
(Although by all means, you can start your negotiation with whatever is lower).