The differences result from how the fabrics are finished. Don't be misled by what's visible to the naked eye, however. Two covers may look the same on the shelf when they're not the same at all, and these proprietary finishes can significantly affect their performance on the job. The prices may be significantly different as well. A cover's price depends for the most part on the type, quality, and quantity of fibers that make up the fabric.
Some covers use multiple types of fibers, mixing in some inexpensive ones to help minimize costs.
How long a cover will last depends on too many variables for a rule of thumb, but a good one will last longer than you might think. Many painters just throw covers away at the end of the day, thinking it takes too long to clean them up. If you opt to clean your covers, you might consider using one of several recent devices made for that purpose. Nonetheless, how long you use a roller cover depends on your own operation. You have to weigh the costs and determine if cleaning or tossing a roller cover is more cost-effective.