Games do not have firm values, the prices shift. I usually have a pretty good handle on it from experience though.
Few 70s games are worth more than $200 in average working condition. Some are worth less. Standout titles that CAN be worth more include Sprint series, Maneater, Pong (original Pong only), Computer Space, Stunt Cycle.
70s VECTOR games tend to be $600+ in good working condition. I listed my Space War (Vectorbeam) at $600 and received 20 inquiries and only one asked for a lower price.
But it is all very amorphous, prices shift from year to year, and from season to season.
Parts for uncommon 1970s games are almost impossible to find, and most pre-invaders games are uncommon. When you are looking at a pre 1979 game that isn't Sprint 2, Sprint 1, Pong, Computer Space, Atari Basketball, Atari Football, Stunt Cycle, or Sea Wolf then there is a good chance you are looking at a game that had a total production of under 300 and has less than 20 existing copies left today.
With all that SAID, if the games are complete then they are repairable, their were only a couple monitor types back then, and they are easy to get (and SUPER reliable), the power supplies rebuild nicely, and the boards are almost always repairable, and if not, then you can USUALLY find any individual pre-79 board you need (other than the Pong clones), with a bit of looking. Well, clone Pong boards are easy to find, finding the RIGHT one is pretty darn hard. If you have a Pong style game with a missing board it might be simpler to rewire your machine to accept whatever flavor of Pong board that you CAN find.
There are lots of parts out there for 80s games. No problem finding that stuff.