$50 for a tuneup may be worthwhile if you don't have the time and it includes things like truing the wheels and repacking loose bearings (many bikes still have loose bearings in the hubs, bottom bracket, or headset). Some of these jobs are time consuming if you don't have the right tools or expertise.

I agree that tweaking the derailleurs and cleaning/oiling the chain are things you really need to learn to do yourself.