If you're on a budget, buying bags to keep your knitting projects in probably isn't one of your highest priorities (personally I'd rather spend my money on yarn).
I use several different types of bags to keep my projects in.
One. Plastic shopping bags from Borders Bookstore.
Two. Club Wedd organizer from Target.
This bag is really great for small knitting projects. It is made of a thin yet durable material with a snap closure. It also has a strap. So I hang the bag on my arm with the yarn inside and the tail hanging out of the bag. It is great when knitting while waiting for the train.
Three. Hand sewn bags. I have two.
Stitch and Bitch Handbook). I actually use this to keep all my little project bags together. J hates all of my bags so if I can combine them in a bigger one it makes it "look" like I have less :)
I really want to try making one of these box bags as seen on DragoKnitFly's blog.
The things I love about handsewn bags are:
You can customize the size and color and it is another way to channel your creative energies.
Keep your eyes open and you can find some great storage options for you knitting. I've even used a package that came from J's underwear.