My recollection working in a game store back then is they did ok. Not sure how they did for TSR; but they sold pretty decent for us. Didn't they have multiple sets?
I have the 1991 & 1992 factory set. I
think there was one more - or at least a partial set. From what i recall they did lose the company money, and the stopped them to focus on printing more
Spellfire, which was making them money. I mean, looking at E-bay, you can buy a factory set of 750 of the cards for as little as $50. I don't know what you'd
do with them...
The cards mostly used recycle art (as did
Spellfire*), but a LOT of it was colored line art too and think that dropped a lot of appeal as time went on.
The non-recycled art looked like this:
They had multiple sets of the trading card style ones in the 1990s which I think were just regurgitating existing art and IP.
The better cards were the Monster Cards from the 1980s, which were, I believe, all new monsters with all new art. There were two sets of those, as I recall. The business model is a little murky, though, since they were supplementing the Monster Manual but the original monsters weren't also on cards, which meant a DM needed the Monster Manual and the cards to use the new monsters (the monsters in questions tended to be weirdos who would supplement an adventure, but not replace the Monster Manual in most cases).
Today, though, Hit Point Press and before them, Gale Force 9 showed that there's a market for putting all the stuff on cards so that players and DMs only need to bring the cards that cover the content for a given session. My wife, for instance, loves her wizard spell cards and won't play without them.
I have those old monster cards, they were given to me by the old owner of a beloved FLGS shortly before he passed away. There's at least two monsters that made their first appearance in that set, though I don't have them handy to remember what they are (I think they got printed into either the Fiend Folio or MM2, can't remember which).
* There was also the
Blood War card game TSR did a little later, but I think it had a lot of unique DiTerrizelli artwork and not a lot of recycled art. I have some of the cards in storage, but I can't recall playing the game.
<Edit:> Oh, and lest it be forgotten, during 3E there were cards that came with the D&D miniatures. They had the "fast-play" stats on one side and the full 3E stats on the other side. I don't think it was until some time during 4E they stopped packing cards with the minis, if I recall correctly.
