Amazon often has good deals on reconditioned roombas in their outlet:
http://www.amazon.com/s/ref=nb_ss_k/002-9352899-2591243?url=search-alias%3Dgarden&field-keywords=roomba
Just make sure you get the one that plugs itself in to recharge. It's worth the extra money.
Warehouse stores will also often have them on sale.
Thanks for the info.