Generally airlines calculate each bag at 30 lbs I believe. So with that in mind, I'd say its reasonable that if its more than double that weight, then the price should at least double for whatever their regular checked bag fee is.

It's an interesting thing though. Ever since fees to check bags came into play, people have countered by either buying massive suitcases and jamming them with their entire closet. Or not checking a bag and trying to fit all their crap in their carry on. So now instead of having 200 normal sized bags in the cargo hold and plenty of overhead bin space for everyone's actual carry ons. You now see 100 ridiculously sized heavy bags in the cargo hold, overhead bins jammed packed, and a handful of unlucky pissed off pax because they were forced to gate check their crap.

Basically, sucks for the pax, sucks for the GAs/FAs, and sucks for the rampers. But...lines the pockets of management!