Diverted at DFW, first class ticket becomes economy

Original tickets was from my small local airport to DFW and then YVR. Got to DFW a couple hours late because we diverted to OK for fuel due to weather issues.

Instead of flying direct to YVR now, I'm flying to PSP then YVR in economy.

I fly first class not because I'm wealthy, but because I'm overweight and have bad hips. Plane seats make me hurt, so if I have more room, it's a bit easier, plus I don't feel like I impact other passengers that way.

Obviously, this last minute flight in economy "looks" expensive. How do I know the cost that it would have been if I'd booked it 2 months ago? Or maybe more to the point...how do I determine what a fair compensation would be for the downgrade in class?