In addition to everything Lori said:
Research the EN13537 ratings for sleeping bags. Except for a very few top-of-the-line sleeping bags (such as Western Mountaineering, Feathered Friends, way out of your budget unless you find a used one), you can't trust the manufacturers' ratings, which (if not EN13537 tested) are pure fiction dreamed up by their marketing departments. A good writeup on EN13537 standards is
here. As a cold sleeper, you want the "Comfort" rating, ostensibly for women. (Not all women are cold sleepers, and not all cold sleepers are women, but that's beside the point.) While there's a lot of criticism of these ratings, they're still far more accurate than manufacturers' marketing departments!
I've found that returning anything to amazon.com is a major hassle, involving among other things inflated shipping charges (you have to use their shipper). I would not shop there for anything I might possibly need to return, and sleeping bags and pads are definitely in that category. Try REI, EMS, backcountrygear.com (a sponsor of this site), backcountry.com (no relation to the previous). You'll get a lot more information and far better service. Backcountrygear.com even had a staff member with my (ample) hip measurements crawl into the sleeping bag I wanted to make sure the bag was big enough!
One bag you might look at, since you're on a budget, is the Kelty Cosmic Down. It comes in 20*F and 0*F versions and is EN13537 rated. It isn't as light as the high end down bags, but it's sure a lot lighter than the bags you're looking at! This bag is generally available at most high-volume sports chain stores such as Dick's and Sports Authority, often at a discount. It's tje recommended bag for Boy Scouts and others on a low budget.
You can increase the warmth of a bag by wearing more insulating clothing inside it, or using a vapor barrier if it's below freezing (the VB has to be under your insulating clothing, though). Just be sure the girth of the bag is big enough that wearing puffy outer clothing inside won't compress either your insulating clothing or the sleeping bag loft.
I agree with Lori that the warm pad is extra important. For 20*F (actually the Celcius equivalent), the EN13537 test uses a pad with an R value of 5. That's a lot warmer than most popular pads on the market.
Have you read the excellent articles on
the home page of this site, left-hand column? They will help a lot with your gear selection process.