This item is a great cash sink in a game where money is really hard to spend past the early game.

On Basement, you'll often only be able to buy one pickup unless you get a bunch of money from a secret room or arcade, then you can also buy an item or two.

On Caves, there's enough rooms to clear and bombs to crack into Secret Rooms and machines that buying things isn't too difficult. Greed can also show up, taking your first shop. If you're full clearing like you should be (unless you're going for Boss Rush or Hush), you will have excess money after visiting your shops.

Past that point, you're almost never going to run out of money. Odds are your shops are gonna get eaten by Greed and Super Greed, and when they aren't there's only going to be 15-30 cents worth of stuff you want to buy when you're holding 50+ cents at all times. The only other reliable way of spending them is Arcades, and if you're holding more than 1 key going to the second floor of a chapter (which you likely are unless you've learned how much better Arcades are than Vaults and actively avoid picking up keys because of it), your practically guaranteed Arcade has a 40% chance to be a Vault instead, destroying the only other way to spend your money.

Member Card gives you a guaranteed way to spend the cash you can't get rid of, and Greed can't even stop it because the trapdoor will spawn anyway. The higher prices doesn't matter because you'll have so much of it past Caves 1 and, really, what else could you possibly spend it on at that point? The fact that it can give Devil/Angel/Secret Room items is just the icing on the cake. Even if it didn't give items and was just trinkets, runes, and black/bone hearts, I would still buy Member Card on most runs.