The BG500 is what you get when you put a BG800 in a room with a BD600 for a few days...
Its basically a BG800, but with the SD-187 tubes like the 600 has and many other non-Barco models. I've always considered those tubes to be nice spunky smaller tubes that can make a decent picture if you don't have too big of a screen.
There is one thing to watch out for when buying a 500 though. Some were sold with limited capabilities as a direct replacements for some older Barco's. If the projector has the built-in keypad its probably okay, but what they did was leave off the keypad and made you use an RCVDS to control the projector, and they also only installed a quad decoder that could only be used with RGB inputs.
I would almost say that for $500 you should be getting fairly nice tubes. Have you seen the level of wear/burn yourself with the lenses removed?
Mark