In addition to the lift kit, you will need a new pitman arm (about $60), 4 or 6 longer shocks (not sure on price, but figure at very least $30-$40 each) and of course whatever hardware parts are not included. Several sources sell a 6" lift with new leaf springs, shocks, and pitman arm for around $750.
Here is just one example:
http://store.4wheelingplus.com/proddetail....25BH&cat=63
I would say figure out exactly what additional stuff you need, check prices, and do the math for yourself, but you probably should try to get the price lower. Specially if it is a used and not a "new" kit.
Edit:
When you do the math, don't forget shipping charges if you order stuff over the internet.
Second Edit:
All of that stuff I wrote is assuming that $500 kit is not one of the more pricey kits from someone like Superlift, Skyjacker, BDS, etc. If thats the case, or if its got extendend radius arms that would make me reconsider the advice I wrote.