I have no idea what they're worth, but I do remember the S-DeC being the foundation for a book called 'Adventures with Electronics' by Tom Duncan in the 1980s. They were quite readily available then, and even sold by people like RS for prototyping use. There was a plastic piece which slotted in to one edge which had three teardrop-shaped holes in, into which controls like potentiometers, switches and variable capacitors could be mounted. I think I've still got my S-DeC and I don't need another one at the moment
Duncan's followup book, 'Adventures with Microelectronics', used a 0.1" pitch 'Verobloc' type prototyping board of the sort that's still popular today for Arduino projects and the like. I've still got that one too...
Chris