As has been said, a lot of the more mainstream brands, Coleman and others, make a pretty fine product that's more than capable of holding up for years of normal use - Slumber Jack is another good brand to look at - same parent company as Sierra Designs and Kelty - I have a 0 degree Moonstone down bag that was worth every penny - but haven't gotten rid of my now 15 or 16 year old synthetic Slumber Jack "Everest Elite". Great bag, I've woken up many mornings with frost on the bag and frozen solid water bottles and been surprised at how cold it was outside of the bag. Once even had my Siberian Husky crawl into the bag during the night. You know it's seriously cold when the husky wants into the bag.
Also check Campmor and Sierra Trading Post - that's all first quality new gear at "used" prices.
And one more jump onto the bandwagon - I've never had an REI bag, but my REI tent is creeping up on 15 years old and still works great.
But as important as the bag - I'm sure you know this - but make sure you get some separation from the tent floor. I'm old school so I still like my Therma-rest - but I hear there's lots of new tech mattresses on the market. No amount of insulation is going to do you any good when your a** is frozen to the ground. DAMHIK.
One more thing to keep in mind, especially with the less expensive, bags, go for a bit colder rating than you think you need. If 20 is as cold as you go, shop in the 0-10 degree range. Better to have to unzip the bag once in awhile then find out the temps. dropped more than you expected.