The importance of getting your boots ( best with custom footbeds) fit by a qualified boots fitter cannot be over emphasized because it will maximize their performance and comfort. If you buy boots on the net and then go to a boot fitter, you may end up spending the same or more than if you bought them from a shop and got the fitting as part of the price. So make sure you know the ultimate price of your boots.
There are many good mid-priced boots, but they often do not have the adjustment features of the high end models. Things like adjustable forward lean, cuff canting, and flex adjustment allow you to better tailor the boot to your body/foot type and style of skiing. So make sure you consider all aspects of a boot, and not just the initial fit.
Obviously, price is a concern, but your boots are your connection to what is going on, so they are the most important ski equipment you can buy. You are much better off with good boots that are set up correctly and used skis, than the latest skis and ill fitting boots. Good fitting boots will make you ski better and longer, and be happier than anything else you can do.
If your feet hurt, it does not matter what else is going on, you are not having a good time.