Cheaper foundations, as a rule, tend to come in a smaller range of colors. An option to deal with those limitations is to mix foundations in order to make a custom shade . . . but if the stuff you need is lighter or darker than the lightest or darkest end of the range, then it's not going to be possible to mix something that works! I watched this Lisa Eldridge video recently about how to get the right shade when you're super, super pale, and one of the things she recommends is FACE Atelier foundation in Zero Minus, which is a white foundation you can mix with the one you have to lighten it up. They also sell Zero Plus and Zero Plus Plus to darken your makeup, and Heat to warm it up, if you find that most options are too cool for your skin. They're not cheap, which is why I am really, really tempted to pick up a couple from HauteLook right now (invite link). Definitely Zero Minus, and maybe Heat? A lot of colors are too cool for me, but Heat might just overcompensate. I'm not sure.
These FACE Atelier foundations are $21-30 on sale, which is obviously not cheap (but much cheaper than their usual $34-48 price tag), but buying one might be more cost effective than buying 5 different foundations that you use a few times and then give up on because they are just not right. You can mix them with any silicone/water-based foundation. What do you think? Worth it?