As far as my personal experience with the two XB's I have go, they are a refurbished pair of 500s I bought not much before their discontinuation and a new pair of 600s. In concerns to aesthetics, the 600s (brown) are more attractive than the 500s, but I must admit the silliness of the 500 pads is something I very much enjoy. I can't say anything about construction quality because I'm certainly no expert. Both pairs are out on loan currently so I can't go into detail, but I find I just enjoy the 500s more, altho it was a difficult decision to me.
Either way, neither of them are probably in the OP's price range currently. I'd suggest the JVC S400 myself, I find the bass quite pleasing, and can be had for under 30.
Oh yeah, as far as the original $20 price limit, I haven't gotten a chance to hear them, but maybe someone else can speak up on if the Panasonic HTX7s are any good. Amazon has the red version for 19.95.