Set: Base
Trash a Treasure card from your hand. Gain a Treasure card costing up to 3$ more; put it into your hand.
Cost: 5
I'm actually a little bit unsure putting a trasher this low. I don't really know if you can consider it a "true" trasher, it never decreases the size of your deck, only improves what's in it, but I guess a lot of cards do that. The problem with this one is that it's simply too slow and expensive for what you need it to do.
There are some nice things about it - I guess the nicest is that it trashes a Copper from your hand without taking away from your buying power for that turn. Sort of. Ultimately Mine works out to be a terminal Copper, which is both good and bad - it's bad for the obvious reason, but it's good because most trashers are actually far worse for your current hand. The thing is, with trashing, it's very nearly always better to sacrifice your current hand to get better hands in the future. Other trashers carry better benefits than this one. Some trash more cards, some are cheaper and thus make better opening buys, some cycle your deck faster or whatnot, like Upgrade, and some are far more versatile in what they can gain, such as Remodel or Expand.
Trashing a Copper to gain a Silver, or a Silver to gain a Gold, is actually not that great. It's good, but my usual mantra applies here: Dominion is a fast game. Mine trashes one card and gets a slightly better card, one at a time. Trashing a Copper to gain a Silver, in fact, is barely any better than simply trashing a Copper and gaining nothing at all, at least in the long run. It's better for your current hand, but you're not really going to reap the benefits in the long run as much as you think. Most Dominion games just play too short for you to play this card enough for it to matter. At a cost of 5, I'd rather have something like Trading Post, which can trash two at once, or Apprentice, which trashes one at a time but can be used for amazing benefits late in the game, which Mine simply cannot do.
One thing to note is that it does synergize kind of neat with Potions - in a pinch, you can use a Silver to gain a Potion, or use a Potion to gain a Gold, depending on what cost you need at the time. It's sort of a niche use and not a great one but it is something. I also like Mine better in games with special treasures, such as Loan, for extra versatility, and I also like it a bit better in Platinum games, where the game tends to run a bit longer, and sometimes getting up to that 9 plateau is hard without a little bit of help.
It's not really a truly bad card, I do play it from time to time, it's just not quite good enough is what it is. It's just a hair too slow for what you need it to do. Deck improvement is a strategy that demands speed in what it does and Mine, as a terminal that trashes only one card for a very minor benefit, is rather clunky to play.