Well, that's an interesting way of doing things... I've seen places where people have suggested a second upgrade for things, but it was always shot down on the basis of being overpowered. (I mean, how much more efficient can you make something like Damselfly or Thunderbolt? 3/3 for 0, or a 7 damage spell or something? :p )
This would give people another task to work on in the endgame when they are near a trainer edition. Mind you, they could just keep buying and selling the 90% standard upgrades until they got the 10% one, but it would still take a lot of electrum to do.