"Allow" and "price reasonably" are two separate questions.

You should wonder first how many airlines have upgrade costs on even their own metal that make upgrading worthwhile. Keep in mind that upgrading (roundtrip) uses half already as many miles as an outright award, and so if you have to spend a lot of $$$$ to be able to upgrade, you have to weigh the $$$$ cost against the double mlies for an outright award.

Meanwhile, the "full fare" Coach that you can upgrade on some partners with often costs more than "discounted" Business! (The difference is in restrictions, "full fare" Coach allows booking at the last minute, and changes at the last minute, while "discounted" Business doesn't. That's why "discounted" Business can be less, sometimes a lot less, than upgradeable "full fare" Coach.)

Seems today that half the airlines only allow upgrades from "not so discounted" fares, and the other half that alow them from discounted fares have ever-escalating "co-pays" that are required to use the miles for upgrades. And thus mile upgrades are getting less valuable (unless you don't need to worry about the $$$$ cost yourself) than outright awards.