Forget about the world of upgrades and whatnot. Just pretend that your only option is to pay cash for the upgrade. there are no other benefits except first/business service. Exact same restrictions and mileage awarded versus coach. How much is the upgrade really worth to you?

For examples:
How much above coach would you pay for a CO 737 first class seat one way from IAH-LAX?
how about an AA 777 upgrade one way in business from ORD-LHR?

You can assume that you have your choice of aisle or window wherever you sit and exit rows are not available. It is very early so it is unclear how full the flight will be.

I'd pay about $20 for the first and $75 for the second, but I'm probably much lower than most.