I never save the brine after I use it.
For one thing I would worry about it spoiling if kept for weeks and another is that unless I was making the same type of cheese over and over the PH would probably be much different, between different varieties.
I normally use canning/pickling salt to make my brine and it really is inexpensive...like $3.00 for 5 lbs.
Since I brine in a tubberware container that is only slightly larger than the cheese, I literally get by with 2-3 cups of whey and probably less than a 1/2 lb. of salt.
Since these tupperware containers have a tight fitting lid, the brine flows over the top of the cheese which completely submerges the cheese.
As for brine used for wiping (like swiss) I do use a saturated salt water brine.
The reason is because this process sometimes goes on for weeks and even if kept in the fridge I would be worried about the whey-brine spoiling.
As I said, by using this method I've never had a problem and it's nice to know that my cheeses aren't melting during the brining process.