Here is how it works with bags- cheap, warm, light-pick two of these. Anyone who says you can get all three has no idea what they are talking about. Example- a down bag is lighter, but may cost twice as much as a comparably rated synthetic bag.
Bags sold under the "house brand" of companies like EMS or REI are good value. Something like this should work for you-
http://www.rei.com/product/798889/rei-zen-25-sleeping-bag-regular-09-special-buyThey have others on their website as well the ones that are on sale.
Look at our sponsors' websites-on the left side of this page. Look at the outlet sites. For example, Backcountry gear has Marmot bags on sale-
http://www.backcountrygear.com/clearance.html?cat=358Marmot is a brand I would trust. Are these the perfect bag? No, but the price is right. I have a pair of Marmot rain pants, so I'm basing my opinion on that piece of gear. You will find bags and other gear on sale for the holidays, so now is a good time to be shopping. Don't pay retail for anything. Someone will have a suitable bag on sale.
Another option is to buy used name brand gear off Craigslist or eBay, but the question remains, what is a bargain and what is junk? I've bought used gear, but I've been looking at gear and reviews of gear for years, so I have a good idea what I want. But, for a newbie, it can be a real challenge.
For example, depending on where you live, you may see dozens of bags on Craigslist and of course on eBay, there are thousands of bags, most of which are cheaply made and not worth the money. it takes a lot of looking to find a deal that won't disappoint you. Given the deals out there, unless you are looking for something specific, you may as well get a new one.
Stay away from the dirt cheap stuff you see at Wal-Mart and similar stores. Based on reviews I have read on Wal-Mart's own site, the temperature ratings are nowhere near the actual warmth of the bags, they are poorly made and often start to fall apart immediately, broken zippers being a common complaint.