As far as I know Walmart doesn't refill any propane tanks. At least around here they don't refill propane tanks.
But Walmart Stores around here do offer and sell Blue Rhino Propane where you bring your empty propane cylinder and exchange the propane cylinder for a new one and pay the price of the new Propane in the new tank.
If you don't bring your tank to exchange it then you'll have to purchase the Blue Rhino Tank with the propane in it which is more than just the propane exchange with your current tank.
The cost here to purchase a new tank of propane through Walmart without your own tank is $47.00 but if you bring your propane cylinder to exchange it then the cost of the propane exchange is $20.00 plus any taxes.
The cost of propane can change from time to time and from location to location so it could be different priced depending on where you're located and the time of the year.