If a physician prescribes a rollator for my Mom, will Medicare pay for it or for a portion of the cost?

You might also check second hand stores and estate sales. Rollators are pretty common if you don't mind gently used.

$500 for a rollator?? That should come with a driver.

Mother bought hers at Walgreens. It's fine, not super heavy duty, but I am sure she paid less than $200 for it. She uses it when she has to take her 2 loads of laundry (per week) the 50 feet to the laundry room. That's about all it's good for, for her. She thinks it makes her look "old". I think it's better than the walker, b/c when we're out, she always has a place to sit. They often have a little compartment under the seat, along with a basket on the front. I personally prefer she walk with this, as her posture is MUCH better with this than the walker, which she hunches along in. The rollater helps her to remember to stand taller. JMO.

My mom had a prescription for a walker. We went to a medical supply store. She chose a rollator that was more expensive than the standard walker Medicare pays for. She paid the difference.
I agree they will pay for only one walker, wheelchair, hospital bed etc. ever so many years. 
I’m not sure a doctor will write rollator on the prescription. I would check with the medical supply store on how the prescription needs to read in order for the medical supply to process without a pushback from Medicare. They could tell you what the final price would be after the payment from Medicare. 

I just bought a rollator with seat for my mother from Walmart for $55. That particular model has a lot of good reviews. Took me about 5 minutes to put it together. Very quickly and very easily. It's light weight, easy to lift, even with one hand. Here is a link.

https://www.walmart.com/ip/Medline-Steel-Rollator-Walker-Burgundy-350-lbs-Capacity/37617768

Everything well covered above so I will comment on Medicare coverage. Their language is that they pay for the "least expensive medical necessity" so you get the cheapest thing they can find unless you cover the difference. In 2013 a new government program changed what they pay stores to about 50% of previous (for the same Medicare products). This caused about 40% of the Home Medical Equipment stores to go out of business and the rest are struggling. This also causes the stores to buy the cheapest possible equipment from suppliers so manufacturers end up getting cheaper/flimsier equipment made overseas. At some point it will likely come down to two choices- free Medicare "junk" or having to pay out of pocket for better quality items (and eliminating the government paperwork). At least that gives you the choice of getting superior/better quality equipment that suits your needs (which matches many of the above comments). Just wanted to let you know what is going on behind the scenes.

I do know that they won't pay for more than one in a five-year period. Shop around though; prices vary.

I just Googled. Wal-Mart has a wide variety under 200. I would have the doc prescribe a PT or IT eval to see what type is best and to teach her how to use it. 

My dad had a prescription from his primary care doctor for his rollator. He either paid a small amount or nothing, I don’t recall. He has both Medicare and his insurance through retirement, so between them most things are covered. It was nowhere near $500 I do know that. Unasked for advice, make sure it’s adjusted high enough to avoid the stooped over posture the elderly often want to adopt with one. We had this done while being “trained” on it in the store

Just want to add my 2 cents of nursing advice here. Rolling walkers can be useful for people with less significant balance issues. I do not like them at all for anyone who has already had a fall. They require strength and balance to use safely and many people just do not realize they are just not strong enough for them. I can no longer count the number of head injuries I have taken care of from people sitting on them and pushing them backward. Truthfully, it is downright horrifying and has given me a whole new appreciation for standard 4 point walkers.

I echo Amazon. My mom had "misused" her last one -- sitting on it and pushing it backwards, so I had to buy a new one. Medicare indeed only pays on one every 5 yrs. I got a great one for $70 or less on Amazon. If you're a prime member, shipping is free.