Should I tune up or replace bike? This question is about the economics of owning and maintaining a bike.

I bought a brand new mountain bike for $250 some months ago. I ride about 30 miles a week on paved trails for occasional exercising. The cost of a yearly tune up in my area is about $80. 

Should I forgo the tune up and buy a new bike every 2 or 3 years? The bike was bought online and is of similar or better quality than those cost $350+ at REI. Entry level but for my purpose it is fine.
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.