I wanted a walker for my mom's stability when walking. Her doctor had not ordered one for her yet but I started investigating the different types/brands. At a private pharmacy I found a decent selection and talked to the pharmacy manager. This was well over 6 years ago so please keep that in mind.
He stated that Medicare would pay to rent one and Mom would have a monthly co-pay. He stated that the walkers I was looking at would not be covered. They were the 'fancy' ones. I knew she would be using one long-term so I did some math and decided that in the long run it would be cheaper (and easier!) if I just bought the one I wanted for her.
The Medicare walkers are the basic aluminum ones-either without any wheels or the wheels on the front legs only. For ease of use most use tennis balls so that the walker slides easily.
I bought her one with a nice padded seat and hand brakes for safety. I can't recommend one with brakes highly enough.
The pharmacy was able to replace the wheels, $10 each, when they wore out and they do wear out. By the time I factored in mom's rental co-pay for the actual 4 years she used it, it was a lot cheaper to buy a good walker than what I feel are the awkward and unsteady aluminum ones.
If you choose to go the private pay route, make sure she's there for a trial run. Some walkers are heavier than others, more difficult to use. Some fold up easily for storage/transport, others don't. Mom's was adjustable for height-I think they all are. The man at the pharmacy adjusted that for me.