If you're looking for a cheap wig that won't be immediately seen as fake, get one with bangs. The hairlines of cheap wigs always look really unnatural, but bangs solve that issue. If you hate bangs and/or are willing to pay a bit more, get a lace front wig from a wig shop which can be blended in with makeup for a more natural hairline.

If you do buy a cheap wig, you have to sort of 'break it in' to make it fit you--cut the bangs to the length you'd like, trim the ends a bit holding the scissors vertically so they're not ridiculously even, and pull out hairs from along the part so it isn't a synthetic perfectly straight line. You can also dab a bit of foundation along the part so that it matches your skin color.

I got a pretty nice $20 dirty blonde wig on amazon that I've worn from time to time, but it does have bangs and I know some people hate them! It's not quite as curly as pictured, but it's heat resistant so you can style it as you'd like.