50-200 has always been a hit or miss I think. Some people seem to love it and I've seen great results from it online, but most (me included) are not that satisfied with it. I've always been satisfied using it with the Raynox though, seems like my copy performs much better at close range than infinity.
100$ is not a bad deal at all, though. If you don't like it you can always just sell it back in the marketplace at practically no loss. If it's me and i already have the 70-300 though I probably would look at getting another lens instead. Do you have fisheye yet? or maybe a flashgun? I don't know which one your fifty is, if it's a f/1.7 I won't try to get you to upgrade to anything... if it's the f/2 then get the f1.7 or f1.4 with the money instead. Big difference
my2c