You're probably not defining your search parameters well.

Stick to the basics, ie. "1/4" grade 25 balls" and you'll find plenty of sellers like this one selling them for $6.95/100.

Grade 25 is probably better than you need, but getting more than you'll need for coffee money makes it a cheap indulgence.

FWIW, I always bought balls by the thousand, kept what I needed with surplus, and donated the rest to a bike co-op.