Gravel driveways are the least expensive driveways.
However every driveway is made of gravel and then covered with the many choices listed above.
In a way, everyone has a gravel driveway with a finish on top. Gravel driveways unfinished, do hold very well if they are not on a steep hill (which rain will wash away eventually) or in a low spot, always under water.
Gravel driveways are only as good as the gravel used. Some rivers gravel are very sandy and get very sloppy in rain and don't compact well. Most good compact able gravel's are man-made and called processed gravel.
If you have a large driveway project and money is issue, I suggest you start with a good solid gravel base leaving it exposed for a year or more until you can afford the desired finish.
It will cost you a little bit more to have just gravel installed for now and having it finished later, but it will break up the cost and have time to fully settle and serve its purpose. You can also do your driveway project in stages. If you have a very large driveway, finish one small section at a time, step by step to fit your budget.
Price range: $.50 sq/ft. - $3. sq/ft. installed
Maintenance: minimum (except washing away on hills and occasional re-grading and weed control)