I have a beloved 500c with an 80mm lens which I'm afraid I might have to part with . Camera and lens are both in pretty good condition. What are they going for these days?
Check the web. I purchased a couple of lenses within the past few years a reasonable prices from dealers. The lenses were in excellent condition and I am sure they made a profit . I understand they usually offer 40-60% of what they will resell the equipment for. I also traded some equipment to Adorama around that time and was taken. I understand they are trying to improve but who knows? You will probably do the best buy selling to a private party. This doesn't help you much but that has been my experience.
Roughly 400 USD and up, depending on condition and details, such as which back, finder, screen.
A good source of pricing info is KEH.com, also completed auctions on ebay. "Pretty good condition" would probably be what KEH calls BGN