A good back wheel will cost more than a bargain-basement department store bike. The good wheel will easily last for 10,000 miles if properly cared for. The department store bike will be good for nothing but scrap after a thousand miles.

The value equation depends entirely on whether you use your bicycle as a mode of transport or a piece of leisure equipment. For people who potter about in a local park once in a blue moon, a Bike Shaped Object is perfectly satisfactory. Their bike is probably going to rust away in their garage before they actually wear anything out. These cheap bikes are built as disposable items for infrequent and undiscerning users.

For a regular commuter, quality matters. It's worth paying $50 for a pair of Schwalbe Marathon Plus tires, because they're damned near impossible to puncture and they'll outlast four sets of off-brand tires. It's worth paying $60 to get your bike properly serviced, because a breakdown will make you late for work and preventative maintenance works out cheaper in the long run than neglect. It's worth paying $90 for a Brooks B17 saddle, because it'll survive until the heat death of the universe and it's as comfortable as an old pair of sneakers.