http://tinyurl.com/3c9jbko
You are probably going to have trouble finding a cheap bag that is accurately rated. Cheap, warm, compressible - pick two. You don't get all three...
Kelty has a few decent bags around 100, the Light Year has gotten decent reviews. The campmor 20 degree bags are 120-130 and actually good to 30F, not 20, but are frequently mentioned as decent budget options.
I am of the mind that I am sooooo cheap I would rather buy something for $300 up front and use it for decades, which is why I have high quality 800+ fp down gear, and not the heavier, bulkier, colder 650 fp versions. But I backpack year round and use the gear on an ongoing basis. For someone who goes a couple times a year maybe, the 100-200 range bags are perhaps a better budget option, with consideration that the bag can be anywhere from 10-20 degrees cooler than they are actually rated. Part of this is you - your comfort zone may be different, plus, people frequently try to climb in the bag long after dinner, and are often not drinking enough water... being dehydrated and not having some good slow burning fats and carbs in your system leads to being cold. You are not in an insulated house with a heater - all the heat comes from you, and is merely held in by the insulation of the bag. Frequently I take cheese sticks on colder outings to snack on just before bed, and take a walk around the area before climbing in for the night to get the blood moving.
There is a rating system for bags, but not all manufacturers bother with getting EN rating for their bags. Any bag under 300 will probably fall into that category. And since part of the equation of keeping warm is how you prepare and what you eat, this is no guarantee either... it's all very much a subjective thing. The one brand I consistently hear good reviews about and that the bags are actually warmer than their rating is Western Mountaineering - but you will rarely find one on sale and they are quite high quality bags.