This is a hard question, etruax. Because it's hard to know until you have one how much you will use/enjoy it.
For myself, I came up on a deal similar to yours-- the roti packaged with a grill I didn't really want. I was fortunate, though, that the grill was a real POS and it languished on CL for a few days and the seller was trying to move and so he finally considered separating the two items. The only problem was he was somehow missing the spit and forks (how would you lose that?) and only had the ring and the motor. I offered him $20, and he took it. Then I got some aftermarket parts to finish out the system for another $30 (way, way cheaper than OEM stuff from Weber). This was all done sometime last fall.
So is $50 a good deal? I think so, because I use the thing on about half my cooks, and it has really turned out some amazing food. It's lowered my food costs, too, because chicken is now much more tasty and interesting for dinner.
For as much as I've already used it, I'd say I could have paid $75-100. But do I enjoy it more because I paid less? You betcha! Would the food taste even better if I'd have gotten it for $25? Of course! That's the fun of the deal. I paid more for my rotis than the Performer I use it on.
The tough thing is that they just don't come up on CL very often in my experience. But as others have said, if you're wanting to buy new, do take a look at the Cajun Bandit-- everyone agrees that they fit tighter and work better than the Webers. I prefer the standard Weber, though, because I like plugging it into AC than fussing with batteries for the motor. But I also end up sealing up the leaky ring with aluminum foil every time I use it. FWIW.