Those are decent prices for those models I guess. The 651/652 used to be the GRE PSR 500/600 so they have been out a while. I have the 651 and it is a very nice digital scanner. I think it is worth $350 at regular price and at $299 for Black Friday that's a good price.
I have the 436 and 536. Comparable, no. But that doesn't mean that the 652/651 are bad. They are good radios. They are just a little older and don't have some of the bells and whistles that the newer 436/536 have.
I wouldn't waste my money on the 650. If I was going to spend close to $100 I'd get the Uniden BC125AT.