costs paid off as soon as you get the required duplicates
if you got 5 pawns, it would instantly pay the upgrade cost
not sure I understand this, you mean you open a box and the coins are drained from your account and the unit auto-upgraded? That would obviously not work, so there is some misunderstanding here.
Anyway, I prefer if there are reasons to not upgrade everything all the time, even if you have required duplicates. There should be a choice involved even if it is small (low tier commons), and as it currently stands the upgrade cost is one of the only reasons a paying player might dump larger amounts of rubies into the exchange since there are limited uses for coins. It cannot be expected that basic pack alone would carry the weight of the exchange, so having upgrade cost means anyone obtaining units outside of market is also burning coins - including login box units, rating boxes, or any other box. Ideally there are more uses for coins added so upgrade costs could be minimal, but I think they are important for now.
If anything there should prob be more uses for coins. As of now most the people still playing the game are only using rubies to get specific pieces rather then using the coins to get the packs (thereby completely ruining the exchange rate) maybe there should be more packs of some kind that would encourage people to buy them instead, or maybe some kind of faction pack.
All the coins I had the other day I just ended up converting to 400 or so rubies in order to buy 2 gluttonies lol. I don't think this would be considered exactly optimal.
Basically, the idea is that there is some value where upgrading is purely positive for your army possibilities (that is some word salad, but I mean like, if you have 8 pawns, upgrading means you only have 3 to use, but if you have 13, you still have 8 to use if you upgrade once).
So in my hypothetical, that surplus would be automatically paid for (incidentally, this auto-upgrade system would save a lot of time lol). In this case, I would then feel free to experiment with any unit I "have", rather than feel incentivized to not upgrade a unit in order to save a few thousand coins. It would be a real 'choice' if it was like upgrading to get a Vampire+++ OR Angel+++ and I had to choose which one I wanted more, but I don't feel like that with getting like, a Temperance+++, where the choice is between "I wanna try it out, at least" vs. "I want to save a couple thousand coins". Even if the coin cost was minimal I'd feel a bit annoyed paying a couple hundred coins for it, it's like fun vs. money-efficient.
Being another market mechanic, Crystals for upgrades may help if it was like, only for upgrades (and maybe regenerating?). Adding choice in upgrading is nice, I think, since there is limited option to choose between getting the copies of Angel or copies of Vampire right now (you basically get equal amount in basics or ultras), but not like this, because I just instantly upgraded to Vampire+++/Angel+++ as those were both things I wanted.