There were always restrictions on who you could and could not draw in a particular set. The thing that surprised me though, was the pay rate--anywhere from $5 to $8 a card, for full color cards. I thought that if I could do them quickly enough, the pay rate would be fine. However, speed is not always a friend of quality, and I soon discovered the the cards were called "sketch cards" in name only. More often, than not, the publishers were looking for finished artwork, sometimes multiple figures and backgrounds, for $5 a card.
Now, I also received a special "Artist's Proof" card for every ten regular cards that I competed. The idea was that I could spend extra time on these limited AP cards and sell them to collectors, at a premium, in order to supplement the low rate on the sketchcards. I still have all of my blank AP cards. I haven't had the motivation, or inspiration, to do some special sketch editions and sell them. Every now and again I see some of my original production cards on ebay, selling for 10 times or more my original rate. I suppose that's the collectors/speculators market. The thing that bugs me, is that it's usually the weaker cards on ebay, the ones I was happier with are nowhere to be seen.
These days I usually think of doing larger illustrations and pieces of art, based on my own ideas. However, when I find my energy or enthusiasm flagging, I kick around the idea of sketch cards again. Simple little sketches that make me appreciate the ability to pursue my own creative ideas.