You can piece together a sump for well under $400 if you spend carefully.
If you use an old scratched up aquarium as the sump, you might get that part for free. Just make sure it doesn't leak.
To keeping plumbing costs down, look for a Magdrive pump which can be placed directly into the sump. If your aquarium is not "reef ready" (which simply means it's drilled for drains) you'll need to get a hang on tank prefilter/overflow box. Get a good one that restarts in the event of power failure. (I prefer Lifereef or Amiracle. These aren't cheap if purchased new.) You'll need hoses and miscellaneous plumbing parts to attach drain/return lines.
The skimmer is the tricky part. Good skimmers aren't cheap, but if you look around you might find a deal. Expect to pay at least $200 for a good used 24" skimmer. This is a rough estimate. Skimmers vary greatly in pricing and quality. The brands that are "in" will cost more than the brands that are "out".