Can of black spray paint - Wal-Mart $0.96.
2 Grip-It-No-Slip Placemats on Clearance - Targets $0.25 each.
Can of green paint I picked up from the school yard the Sunday after Homecoming. I try to go out to the school the weekend after homecoming. Kids have taken apart their floats and have left wonderful treasures behind.
A little creativity and about an hour of my time total. (10 minutes spray painting it black, 45 minutes painting dots.)
Ta Da! Now every teacher wants to check out "my little Pygmalion"
Options: I have also just painted them a pretty color and used one roll of the Grip-It-No-Slip shelf liner from Dollar Tree to cover the shelfs. ($1.00 per roll - one roll will make a runner down 3 shelves). If you don't have the time, energy, or talent, send the carts to your art department and let the students jazz them up.
Give us your ideas