Google this 'MacGregor Bucket of Yellow 12" Softballs', I bought mine at walmart online but they only list 11" now. But you can find them for 60ish. I have 4 buckets for practice and they work great.
We buy leather game balls even for practice. You can usually get them for about $45 a dozen or less if you look. We tried some "practice" balls along the way and found that even though they were a very respectable brand the balls did not hold up at all. We do practice hard but the games balls seem to hold up better and in the long run save us money. For reference we buy about 8 dozen balls for practice per season.
The best prices I have found for balls softball.com. Their service is excellent and they have a good variety. their service is quick. I always just get the basic ground service and have always gotten everything on about 4 days. Just ordered a dozen 11" Rawling leathers and after a discount on the website paid $60(including shipping). Don't buy Powerbolt or Adidas from Dick's yhey don't hold their shape if you use them for bp.
I usually buy about 4 dozen balls a season. We get the leather dudley because those seem to be what most tournaments in the area use. About half the balls get trashed over the season but the other half hold up fine. I usually get them at west coast (When they have them in stock) or softball.com when they don't.
I used to buy crates of used balls on ebay but I found it a mixed bag, as half were recovered from tournaments and waterlogged to one degree or another. Isn't worth saving $100 for a few dozen balls for a sub-par product.
For the last two years, I have purchased used tournament balls wherever my DD would be playing that weekend. That is if that particular TD was selling them. The balls are brand new when they are used for tournament play. When I purchase them, they almost look brand new, there are some with some heavy scuffs on them, but their perfect for practice. I usually pay about $1.25 to $2.00 for each ball.