The high cost is for grinders. Gaining 20-30 but even 40-60 in case of mastery win is crap if ONE SINGLE UPGRADE costs 1500 coins.
I could agree if those upgraded cards would be AWESOMEly better than regular ones, but they are not SO better. Many times they are only the underpriced version of the original card, that could be great if you want to play that card in the first few turns, but it's quite useless in mid and end-game.
Who cares if improved steal costs 1 quantum less than steal?!? Shoud I pay 1500 coins for ONE FREAKING MANA?!? Are we crazy or what?!? ??? ??? ???
I've only improved Vampire Stiletto to Vampire Dagger, only because its effectiveness is DOUBLED. I'm currently looking for another card to upgrade and i don't see anything that would justify this odd price. Maybe Towers, but unless you have a good number of them, their impact on the game will be very very very little.
Once more, we are talking about paying 1500 coins only to get ONE mana more, UNA TANTUM. If those Towers would grant 2 mana per turn, they would be cost-effective, but they are NOT.
The upgrading system is grinder's dream, but i don't want to play SIX/SEVEN HUNDRED matches only to get ONE DAMN CARD upgraded, AND to see this "fabolous" upgraded card not having a significant role in the game.
Those costs are ridicolously high.