It depends on wheather you are buying for yourself or if you buying these items and plan on selling them off to make a profit. If you are just buying for yourself, then the answer is simple! Any item is worth what you are willing to pay for it, period!
If you are planning to sell them for a profit, then you have to know what others are willing to pay for it, and how much profit you want to make off it. I would conservatively guess about 7-10% profit. So, find out what others are willing to pay for it, and subtract 10% from that price, and that would be your max bid!
PS- just a reminder: condition is everything! Junk will always be just that.....JUNK!