Standard big box store sleeping bags are both too heavy and too bulky. Backpacking brand sleeping bags are nice, but they generally run well over our budget. Some budget minded folks instead go with poncho liners or surplus wool blankets, but in my experience, neither are a great substitute for a decent sleeping bag.
There are luckily some good options for sub $40 sleeping bags, and of those, the USGI Patrol Bag is amongst the best.
The Patrol Bag is the lighter weight bag from the military sleep system, good down to a claimed 30 degrees. Like all sleeping bags, the rating will vary depending on real world conditions--sleeping surface, what clothes you're wearing, individual tolerances and so on. It's a good rating for a bag to use in a wide range of conditions--comfortable enough for summer weight, sleeping indoors, and with enough warmth for sleeping outdoors in a variety of conditions. Large portions of the 'states don't get much below freezing all year, and if you do find yourself in really cold conditions, a good shelter and warming fire would help keep you alive.
The Patrol Bag is a mummy style, but not so restrictive that you can't move around, sleep on your side or skip using the mummy hood. I'm not a small guy, and the Patrol Bag has plenty of space for me; big muscle-y Marines use these too, so you'd have to be a pretty big boy to have fit problems.
Because its issued to big ol' Marines, known for their penchant for destroying anything, you know the Patrol Bag is going to be a tough piece of kit. The zipper is the heaviest duty that I've ever seen on a sleeping bag--a nice, big YKK zipper. The materials in general are quite good.
Despite the rugged build quality, the Patrol Bag still manages to maintain a backpacking-friendly weight at under 3 pounds.
My personal Patrol Bag set me back $27 shipped from an eBay seller, and you can find them on Amazon for around the same price point, maybe a few bucks more. They are a great bag for the price.
As another plus, the Patrol Bag is part of the Military Sleep System, so you can purchase the other components to get down to colder temps if you desire--black Intermediate bag, Gortex bivvy and stuff sack. The complete MSS is rated down to -30 degrees for those living in the frozen north.
If you do live somewhere extra cold where a 30 degree bag will struggle, you probably want to pony up the few extra bucks the black Intermediate bag instead of the Patrol Bag--the Intermediate bag is rated down to -10 degrees. They typically run about $10 to $20 more than the Patrol Bag.
Cost: Varies depending on the seller and the price, but should be around the $30 mark.
Bank: We'll stash the leftover $10 in our savings, bringing that total to $20.36.
If you've already got a good sleeping bag...
An extra sleeping bag isn't a bad thing -- they're a great thing to have stashed in the trunk. If you're squared away on bags though, how about a sleeping pad? While sleeping on the cold, hard ground may seem like the "tough guy" thing to do, it's not only uncomfortable, but sucks heat right out of you. A good, backpacking weight sleeping pad is a handy thing to have, and makes your night's sleep more comfortable.
I use a ThermaRest ProLite that I've had for 6 or so years. It's a bit thin for my tastes, so I fold it in half, giving my torso a double-thickness of mat. Not a pillow top mattress, but better than the ground.