Almost every Android phone is structured differently internally, and because of that, each version of Android for each phone has to be altered to make it work. Each phone has to be updated by the manufacturer, tested for bugs, have the bugs fixed, tested again, pass the test, sent to the carrier, tested for bugs, pass the test, and then sometimes the carrier adds on their own programs. Then it has to be tested again, approved, and then sent out to consumers. That is a simplified version of it. For many carriers, they don't update the Android OS version more than once, twice if you are fortunate, and if 3 or more, then you lucked out.
However, even if that was possible to just update the phone using the regular Google OS version, the hardware in almost every phone is different. The phones that cost more don't really cost more because of the version of Android, but because of the higher quality internals, like processors, memory, resolution/touchscreen, radios, battery, and many other factors.
I'd recommend doing research into any phone that will fulfill your wants, and then look to see it's history of updates, and whether there are more updates coming down the line.