First, I'd like to mention that sell prices are roughly 2/3 of the buy prices.
If this was implemented, pillars/towers would be 560e to upgrade, and are one of the most important cards to upgrade. At the other end of the spectrum, Miracle (the highest selling unupped card) which sells for 159e, would be 2090e to upgrade, for a relatively small benefit (-3 casting cost, which is, I suppose quite large, but not when it destroys all your light quanta anyway). Titan (149e) would be 1990e to upgrade, for only one extra damage.
A different way (to the current one) of charging for upgrades would be good though.