It's funny how eden gear and boxes made almost all gear worthless.
To an extent anything that existed as a normal without special set bonus gear is deprecated severely with the Eden Gear. Things like Shoes, Boots, Muffler, Chainmail, Silk Robe Hat etc all are outclassed significantly by the Eden Gear. The items with set bonus like Morpheus set do hold up a bit better with their unique effects but the lack of a slot and high defense on those older set gears makes them a hard sell on use now. I would love to put those sets through a revamp and release V2 of them, but that is a very long term project.
the better question is what would gms want more? a potential slight zeny sink or a possible kafra item?
Really if it were a perfect world much of the cards would go up in smoke, including MVP cards. Fewer of them makes the server more apt for players hunting rather than trying to trade for them. Of course I know what my response would be if my Maya Purple Monocle, +4 GTB Guard +5 GR Cotton Shirt and Deviling Hood suddenly vanished would be, and I think players that have those would also be outraged.
Realistic a zeny sink works but the problem with making it expensive is why even allow the low grade cards to even attempt it if the values are they skewed? Wouldn't it then make more sense to only allow decarding of the few dozen cards that have "high" value. But if We do that we have players that want to recover certain utility cards that aren't on the market but also aren't very valuable too but can't.
Making it a "service" also has benefits in the quantity certainly would be lower, and can be done as an event type give away as well. Raffle, Box, extreme Merit Badge Turnin, grand prize on daily item lotto etc. I don't think players will be happy if it is only expensive, or if it is only a purchase thing, some consideration in how to implement it is important because it is a decision that will last a long time so doing it right is key.
If Gear and Cards were acct bound this would be a no brainer : )