Once again I'm going to demonstrate my complete ignorance of the value of toys and cheezy keyboards from the 80's and say that a few minutes ago I bought a near mint-condition Casio Rapman on eBay for 25 pounds. Not a bargain by any definition of the term, but I've seen these sell for 70 pounds on eBay in worn or broken condition (missing keys & buttons), especially when there is a bidding war due to high demand. This one I just got is one of the cleanest I've seen in almost a year, so to me it was money well spent.
Here's a couple of pics of the one I just bought:
Ugly little bugger, aint he? As you can see, it's in pretty nice condition and nothing is missing... except for the Casio EM-4131 mini microphone, which shouldn't be too hard to find anyway. I think it's pretty pathetic how a rare toy like this can easily part a fool (like me) with his hard earned cash!