Well my $25 and $50 bikes have required exactly zero maintenance. I oil the chain once a year, and that's pretty much it. Originally these bikes cost about $200 new I think (Raleigh Ozark and MTB Trekker), so they're fairly decent bikes that are about one step up from the Walmart models. Note that I only ride a couple of times a week, and not every day.

To be honest I think once you spend more than a few hundred dollars you're just paying for fancy suspension and fancy brand-name. I could of course be wrong, as I've never bought an expensive bike.