The variable upgrade is based on the cards. Not all upgrades are even, despite being balanced, against each other. Some get ability upgrades, while others just get a small casting cost reduction. When I said variable, I meant almost mirror to how the upgrades already are, they will just be done differently when compared to the actual upgrades.
And when some come out better than their normal upgrade form, it's mostly a matter of opinion since all the upgrades will differ from the current upgrades. Some might seem better while others might seem not as good.
If they are meant to mirror how the upgrades already are then the upgrade cost should be equivalent. (although you might have ideas for multiple currencies?)
No, they probably won't mirror the casting costs, ability costs, attack, HP. They will change based on whatever the new upgrade is in a (hopefully) balanced way. Such as, Pulverizer might not get it's Ability cost reduced, instead something else might happen and the casting cost would reflect that. Just an example, lets say Pulvy can destory two cards with one activation, either the ability or the casting cost would changed appropriately. (that's not what the change will be I'm pretty sure, just an example)
The upgrade mirroring is in the overall balance and not the exact specifics of each card. And as such, the upgrading price will be somewhat dependent on the initial buy cost of that card. So an Underground Pillar will be cheaper than an Underground Crimson Dragon.
I agree with OldTrees
Elements has a formula to determine card cost. Maybe the Black Market could work like Tyrant's pack system.
Like trade upped card to get a pack of underground cards.
Love the idea though.
I don't recall seeing such a formula. I know of the one OT has devised, but I do not believe that is the actual(if one exists) formula for it as governed by Zanz. I could be wrong on that though.