I have found that the website for Hallmark Scrapbook will have the cheapest prices for all of the cartridges. Last Christmas, I bought 12 cartridges and only spent about $150 (qualified for free shipping
If you watch that site, they will have several specials on the machines as well.
I purchased my expression machine off ebay for about $200
and it came with 2 cartridges. I had the baby (original) machine before that -- I bought that one at Sam's Club for $225
(but it came with 3 cartridges, 4 cutting mats, the tool kit and 4 extra blades).
you just have to be patient with the purchase ... when the timing is right -- you will know