This figure is one of my favorite space toys! I didn't grow up with Major Matt Mason, so I imprinted on a beat-up Billy Blastoff that I found at a flea market. That one didn't survive my childhood, so I picked a spiffy, operational version off eBay much later on.
It's a clever format for a toy - let the figure itself power all of the accessories (even his laser pistol). Billy's backpack is motorized, so you simply plug it into any of the vehicles in the line, and away you go! There were several different conveyances to choose from, as well as a full-blown moon base, and Robbie, his robot buddy. Billy was also available as a fireman and a scuba diver (there may be still other variants), each with still more accessories. As far as I know, the astronaut is the most common. It makes sense - once you come up with a catchy name like Billy Blastoff, anything else just isn't gonna cut it!
These aren't as popular as the Major Matt toys, and they're also easier to find in good shape (no wire armatures to break), but some items can get pricey, depending on rarity and how difficult it is to find complete. There's an eBay auction for a decent-condition Billy with several vehicles that's currently at $45.12 + shipping.