There was quite a discussion of the 2 Mattel D&D games a while back:
viewtopic.php?f=1&t=11135&hilit=mattelThe little handheld comes up for sale quite a lot and I think you can get one in average working condition for around $20-$30.
I think though there is a crossover here between RPG/D&D collectors and electronic game collectors. For a mint in box, I think you be looking at $150 + with the latter driving the market.