If you buy at the right time, you can get an entry level foodsaver for around $40 - with a trial of bags. You will end up using the sealer for more than just immersion cooking. You can buy in bulk and freeze. You can make large batches of soup or chili and freeze. I lay them flat when freezing to make "soup books". If you get all the air out, they will never go bad. You should get the heavier duty bags for sous vide and freezing (freezer duty - now called gamesaver). Go with the rolls - they are the most economical. Also, the bags go on sale every once in a while - right now they are buy 3 get 3 with free shipping. You can use a coupon code on top of that. Kroger by me ends up having them on closeout a lot as well.
Any air in the system will make your sous vide bags rise to the top. You will end up with uneven cooking due to your bobbing bags. Without the air, the meat is basically the same buoyancy as the water and will sit under the water while it is circulating - not on top. Also, you never have to worry about if the zip lock on the bag leaked and ruined your $100 worth of steaks due to water infiltration. I also question how well cheap bags actually deal with heat. They aren't intended to be sitting in 140 deg water and I doubt any of their chemical leech testing incorporated high heat for long periods of time.