You can get seasons six and seven from
http://www.dvd2blu.com for $14.95 each, and season eight for $19.95. Free shipping on orders over $35. The whole process takes far longer than ordering from Amazon, but if you don't mind the wait, then it will end up being much cheaper.
DVD2Blu requires you to send in DVDs, but they pay shipping and it can be any non-burned DVD, including demos, special features discs, cereal box and magazine freebies, etc.