I bought Gmod 10 not to long ago (with the current exchange rates it hardly broke the bank). On its own it nice to muck around with for a while... then you discover the mods. Gmod 10 with mods like Spacebuild (build spaceships and fly in outer space), Lifesupport (adds a new level of options as your ships need resources to keep you alive), Wire (THE Gmod mod! build automatic homing barrel throwers, computer controlled vehicles, the possibilities are endless), the list goes on.
In singleplayer you can waste hours and hours in Gmod. Find a decent mp server and you can waste even more. Not bad for $10.