I have seen prices for 500's be anywhere between £20-£100. Depending on condition and how cheeky the seller feels. Also depending on what goodies are thrown in with it. So £50 is pretty much on the nose for sale prices. I wouldn't expect them to increase in price as drastically as you are hoping for.
However saying that I have noticed since CUSA have started selling their replica 64x that the prices for real C64's have fluctuated. So maybe if the 500x does appear then it's possible that real 500's will increase their value. But again I can't see it getting to the amount your hoping for.
Originally Posted by kipper2k
I think i'll only sell some of mine if they agree to look after it
I am the same. I would only sell mine to somone who is going to look after it and treat it right