It's hard to give specific advice on exactly what sleeping bag you should buy. What I can help with is some general advice on what to look for:
Probably the most important thing. Generally two types of filling will be offered:
Synthetic is typically much cheaper than down. But it's also bulkier, heavier and less resilient. There are also various qualities of down, so two downs are not necessarily the same quality.
Both types may of been "treated" to make them more water proof, etc.
Materials of outer
Much like the filling the materials that the outer are made of will vary in quality, weight, etc. Again this depends on what you want. If you're going up everest and will be living in your sleeping bag for long periods of time you'll want a good, light, strong outer, if you're camping once a year then a cheaper one will do.
Materials of the inner
This basically boils down to how comfortable it is inside the sleeping bag. Again materials vary considerably from silk to polyester. Down to choice and what you want it for again I'm afraid.
How strong is the zip, will it break? If you're going to use it a lot you want one that's quite resistant, etc.
A mummy shape (or other hybrid on this theme) will be more expensive as it's more difficult to make but it also minimises "dead air" and materials, when compared to a bog standard rectangular one.
Range of temperatures
All sleeping bags should come with a range of temperatures that they are made for. Look for one that meets what you expect to encounter. They will also be classified as 2,3 or 4 season. So:
2 season - spring summer
3 season - spring, summer, autumn
4 season - spring, summer, autumn winter
Is it realistic to expect to pay $100 or less for a sleeping bag that
will keep me warm enough for camping in places such as the U.S.
Midwest, Yellowstone, etc. in the spring-fall seasons?
Yes. Most sleeping bags will be fine for 2 season use. You'll want to pay extra to keep it small and light.
The only thing that worries me about these bags are there pack-down
size. I am not really sure what a normal size of a sleeping bag is
once in it's stuff sack ... it would be helpful to have a range.
This is vast, roughly, I've seen sleeping bags that will compress down to about 4" diameter to one's that are about the size of a spare tyre. all of the above varies this and typically the smaller the more expensive.