I didn't buy a pre-made net made for roundbales yet, but I did make my own. My animals gobbled the hay down too fast and I was looking for a way to slow them down and not waste hay.
But I wasn't sure it would really work so I checked around on slow feeder sites for ideas and looked at the price of different types of materials/nettings. I ended up making somethang that has turned out to work pretty good and the animals are really happy that they can munch when they want. After learning on smaller slow feeder hay nets they are quite good at "grazing" on it, and so far it has nearly doubled the amount of time a round bale lasts.
The pics and a descrition are on this thread- making a slow feeder
All in all, it cost me about $30 and I was willing to try that approach fist to see if it would work before spending quite a bit on a pre-made one.