eBay is a great place! I've found all the cineromanzi I have there.
As for prices that's a very individual thing. It seems to vary a lot what people think is a fair price. It also depends on the rarity of the magazine I'd say. I've seen that cineromanzo of CASE OF THE SCORPION'S TAIL up on eBay many times. It's not really all that rare so I wouldn't pay too much for it - no more than 15-20 Euros. But that's just me, of course. Others might disagree. As for the really rare titles (the Polselli stuff etc) you must expect to pay a lot more if you want to get your hands on those.