I buy mine at a farm co-op. 50 lbs for I think $20... it stores well in covered 5 gallon buckets. As long as it doesn’t get wet it works well.
I have 40 regular and 10 juniors but even then I have a lot left from purchasing 2 years ago.
I try to be reasonably consistent in using it. I use it on all veggies, not just tomatoes.