You should see the prices some of the modded SK's are asking, like $250+
Sk5's go for around US$60, Sk1's for a bit less sometimes.
The Sk100 can have all the same mods done to it as the SK5 and 8 I mentioned, so if you were after it for the larger keyboard, it's a good buy. The only difference is that the SK100 has only 2 samples where as the SK5 and 8 can have 4, and they are each the same time length as the SK100's. It's a shame about the small sampling time on these keyboards, but I suppose they were made kind of as a toy back in the 80's for $100, which is pretty amazing considering sampling was fairly new and expensive back then. The Ensoniq Mirage was released only a couple of years earlier than them, and it was about $2000 with a similar sample time and quality.
Did I also mention that there is a bunch of guys working on swapping the Casio's preset sounds with their own?
They've had some sort of success, making up their own sample ROM chips:
Another small sampler keyboard to watch out for is the Yamaha Vss-30, 100 and 200
They do much longer (up to 8 seconds) sampling, much better sound quality and editing features. Unfortunately, none of them retain the samples when you switch them off