How long's a piece of string? If you're thinking of a sump tank, you'd probably break even; if you're thinking of a 10ft. x 3ft x 3ft you would be far better getting it built by a professional. It could save you thousands in the event of a DIY project going wrong. I have many times been called on to put right disasterous DIY tank builds, and have witnessed the ensuing chaos at first hand.