Worth remembering too that even though they are scarce, not many people have the players .. so their value hasn't sky rocketed. I've seen lots for around the £15 mark, which I find very reasonable. They have great curio value!
Similar to Lazerdiscs in that respect... I've seen the early 80's ZSMP lazerdisc upwards of £50. It's big and shiney, but you can't play it.