The reason that the doubles are available for less than the singles is because they cost less for manufacturers to make. They use more material, so it is essentially like you are buying in bulk. The single types are actually custom made, and they take more work. Brands like Kelty and Coleman generally don't worry about this because they figure individuals will never by a double sleeping bag unless they specifically need one. That's where you can really take advantage and get a bag you need for a lot less money.
Don't worry too much about the specific brand name you go with, because all of them offer a high quality options at cheap prices. For example, you can check out what Big Agnes has to offer or you can go with Suisse Sport or Mountain Engineering. Many people will get a double sleeping bag and use that as extra protection against the cold, even if they are not sharing it with another person. This is actually a good idea and it provides a lot of extra padding as well. This is the only substitute for a good mummy sleeping bag in the cold weather.
So if you are thinking about going camping or backpacking in the winter or to a cold area, then make sure to bring along a high quality mummy sleeping bag. You will be glad you did when you feel extremely comfortable throughout the night and wake up feeling well rested and fully healthy. Since they don't cost more money than the standard types, there is really no reason not to do this important step. You will be so glad that you did.
Read more for information on how to get a custom made 2 man tent for less than $20.