There are many monsters people don't have the card for, are annoying to farm for, or are limited bosses that make card farming difficult, tedious, and causes potential user conflict. What I suggest is similar to Chair Gacha where people can roll for a random chair, a Card Gacha where people can roll for a random card. Now I'm unsure if we'd want different tiers "Red Card, Blue Card, Green Card, etc) , but this could be a great way to pull mesos out of the economy much like Chair Gacha does. It could have different pricing for different rarities or odds of new cards. For example it could be 100m for a Gold Card (Boss) Pull, versus say 2m for a red card pull (Snail, Stump, etc) or if the system allowed it different prices for how likely a card is a duplicate of one you already have.
I don't think it will be a good idea. Not because the idea itself is bad, but because - who's rich enough to spend 100m on a card? and why? o.o It's easier to just hunt for like half an hour or so the mob area and get the cards. it's not that rare of a drop that it needs a whole gachapon for it. Assuming you mean gold card = 1 card (and u need 5 to complete a set), it'll be 500m per gold set or like 10m per red set. And to top it all off - as you said, it's optional to even get dupe cards, so it can be a couple bills to get a couple of full sets or even a singular one. Of course, that's my opinion and others may think differently.
I think it kinda loses the point, you shouldn't allow people to do things in game just for money. The fun is that some things you gotta grind for.
