you don't have to put plywood or sheeting down for the metal roof, like bear foot farms said, you run purlins (battens, lathing, strips) along the rafters, start at the bottom and go up every two feet. also it's been pointed out, find your local metal roof people. they will have what's called 'galvalume' a 5v crimp metal roofing. ours here sells first quality for 1.60 a foot, and seconds for 1.30 a foot, and they cut it to the lengths you want! one last thing in my opinion, the corrugated metal from HD is really thin stuff! oh yeah, metal roofing sweats when the weather is cooling down, so you'll want to put some moisture barrier under it to keep it from dripping on your girls. simple plastic from the painting section of HD or Lowes can be stapled in....
all in all, $200 is not a bad price really, but if you look around craigslist, check with people doing renovations, look for anyone tearing down an old shed or barn....lots of ways to save on the cost of materials