What bag you get depends on what weather you will encounter. An alternative to a bag is a quilt and many members here use one. I'm not one of them, so others can inform you about them. Ray Jardine, a famous lightweight backpacker, author and gear maker sells a quilt kit that is quite inexpensive. I would consider that instead of a cheap bag if you have any sewing skills or know someone who does.

Cheap bags, like those sold at Wal-Mart have a bad reputation for not being warm and coming apart. Look on the Wal-Mart websites for comments about them.

Used quality gear is an alternative, but that is only a good idea if you know what you are looking at and know whether the price is right for used, compared to new. Online stores like Sierra Trading Post offer a lot of gear at good prices, often close to what things are selling for on eBay. I've actually seen people pay more than retail on eBay because they had no idea about prices or where to look for discounts. Many brands have outlet websites (REI being one) selling discontinued or overstock items. Unless you are buying cutting edge gear for some special reason, you don't need the latest and greatest. Example-I have a very nice tent, parka and sleeping bag I bought used and paid about half retail for each of them. However, I knew what I was looking at, knew the brands, knew the value and was patient when looking.