4 casters at Lowe's $39.12
http://www.lowes.com/pd_4491-255-403...7C0&facetInfo=

Rubber floor mat at Lowe's $8.98 (cut it to fit on the board)
http://www.lowes.com/pd_320875-73825...7C0&facetInfo=

You'll probably spend $5 on screws/bolts/washers depending on how you want to attach the casters to the wood. You'll probably also spend around $5 on the wood.

Grand total to build it yourself - $58.10