Advanced Auto Parts has 5-pack of 16oz Seafoam Motor Treatment for Gas and Diesel Engines for $34.95 - $10 with coupon code A123 = $24.95 + free store pick-up. I think that is the cheapest I have seen for Seafoam. Good stuff.
http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product__6410004-P_N3233_T
http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product__6410004-P_N3233_T