Should I tune up or replace bike?
My answer would be:

Politely ask the bike shop what you are getting for that $80 tune-up. 

I'm of the same mind as @danielrhicks, $80 sounds a lot for a tune-up, unless they are trueing your wheels (in which case you're paying for the mechanics skills), and replacing parts (in which case you're paying for...parts).

Depending on what they are doing for that $80, I would be trying to do some/all of that tune-up myself - regular cleaning, pumping of tires, checking "stuff" isn't loose, visual check wheels aren't buckled. 

If you don't have the basic skills of bike upkeep, maybe there is a local community centre, or bike club that give free classes on how to take care of your bike...or even ask questions here! :)

To me, if you take regular care of your bike then the big jobs are seldom and cost less.

I'm not saying don't get those regular tune-ups, but maybe you can ask them to focus on the brake cables/wheel straightening - jobs you can't do yourself.