I think I have $160 into mine in stuff I had to buy, including my 250w HPS light (which I bought off Craig's List for $50) and my timers. I had the fans already, had one of the lights already, had flower pots and dirt, spray bottles and humidity/temp gauges and even the box. Everything else I'm just scrounging around for. (I have quite a few things saved up for a hydro grow too but am missing a few key items.)
Tomorrow when I paint the inside of my grow box, I'll be using paint I bought over a year ago when I moved in here and brushes I've had around for a while too.
When I build a filter, I won't be having to buy much there either, I have screen, I have big hose clamps, I have an extra length of new dryer hose and lots of duct tape. lol I think I can improvise on a lot of these things.
Will look for computer fans too. I think I want two or three. Two for exhaust and one to blow. When I seal up the back I'll be able to cover it in white wood left over from another project. I love it when a plan comes together.