sure I'll chime in :) $400 will last you just long enough to buy second hand lights and a second hand skimmer. not counting the tank, stand, sump, ATO, RO/DI, sump/fuge, return pump, powerheads, heater, chiller, control unit, probes, possible calcium reactor, plumbing supplies.
I really REALLY wish that didn't sound so a$$inine but at this moment I am doing a 40b build and the ONLY reason I am under $400 is that I have been scrounging the last 2 years finding awesome deals and trading/selling those pieces and working up the parts I need, and even then I had 1/2 the stuff laying around or I wouldn't even be close yet. If you had a budget of $750, yeah you could do it but it will still be mostly second hand stuff and your rock/sand will need to be found someplace like craigslist where you will inherit all the problems the last guy had.