Buying a bike is like everything else- there are certain responsibilities that come with it. If you buy a battery operated device, you know you're going to have to buy batteries. If you buy a car you know you're going to have to buy gasoline, insurance, and tires, and etc.
When you buy a bike you know that sooner or later it is going to need tires, tubes, and service. If you can't handle the cost or hassle of either, you shouldn't get a bike.
If the $50 cost of maintenance/repair is too high, you can go to a goodwill store or garage sale and look for another bike. Buy one for $10-20 and ride it until it breaks, then look for another and do the same again. Of course you may have to visit a lot of garage sales before you find a bike that's in better shape than the one you're about to throw away...
For most people, the $50 cost of a "tune-up" is either enough of a motivation to learn to do it themselves or enough to put them off bikes entirely. The hassle of shopping for a new bike (and disposing of the old one) every time something goes out of adjustment is more costly than $50 to get the old one fixed. But if you have LOTS of time one your hands, and no money, well, maybe it's time to go shopping or dumpster diving.
You can actually buy brand new "bikes" at Walmart for about $50. Do that a couple times per year and you'll probably never even need to put air in the tires. You can probably sell the used bike (with soft tires) for $20, so it will cost you $30 each time you get a new bike, a couple times per year.
Tuning a bike yourself is NOT a major undertaking. First make a list of the problems that need addressing, then go to the library or surf the web and find tweak instructions (or use your eyes and ability to reason and observe the operation of the parts and figure it out for yourself- it ain't rocket science!).
You can fix 95% of bike problems with an adjustable wrench and a screw driver (cost: $10 new, $2 at a garage sale). You'll need some 3 in 1 oil for the chain ($1- and no, you don't need the $10 per 2 oz bottle bike stuff) but only if you don't like squeaking and/or you ever attempt to shift gears.
Finally, if you want lowest maintenance cost, get an old one-speed bike with fat tires and a coaster brake. All you will ever have to do is replace tires/tubes once every 2 or 3 years, and oil the chain about as often.
TD