If looking for new,kensfish is the cheapest I have found.Not a fancy website,but great prices.If you buy a used one,your buying at your own risk,IMO. http://www.kensfish.com/
I got an eheim 2217. Came complete with all media, etc. Was $134 plus $6 shipping and about $9 tax. Ended up being about $150 total. Not bad considering the used ones were going for $100 - $120 on ebay. I've got Kensfish saved in my favorites now though and will check there when the next need rolls around. I'm sure there will be more fishy purchases...