You need to let us know what you want to use the sump for also. We know you want to house the skimmer there. Do you plan to use part of it as a refugium? how much flow do you want through it? Are you going to add a calcium reactor later? Kalk reactor? external return pump or submerged? If external I would suggest am acrylic tank as you can easily drill it yourself.
I agree with Nick though. No way a sump setup (even with the return pump and plumbing) should cost anywhere near $400.
Also, like Nick suggested, sumps can be as simple as a rubbermaid tub and as complicated as the mind can imagine. You have to decide what you want it to be...then we'll be happy to help you out in design, materials, and costs.
Good luck.