Hmm,
It really depends on the concentrate of your spray. The max rate of captan 80 is 5 lbs.per acre. So at the max rate a 30 lb. bag will cover 6 acres. If one is spraying at 100 gal per acre (about 2x concentrate), then that would take 600 gal to cover 6 acres.
So, a $125 , 30 lb bag would mix 600 gal. So 300 gal would be half that, at roughly $62.
As you can see we’ve made a lot of assumptions (rate of captan per acre, which according to the label has a wide variance, concentration of concentrate sprays - in other words 2X, 3X , 5X, etc).
If I were figuring on a per acre basis. I would calculate it as $20.83 per acre/application, which is pretty much consistent from year to year (at the max rate) from my experience. There are much more expensive materials currently labeled for use on specialty crops.
Indar has run roughly $12 per acre/application in the past, but it is much more expensive per bottle (over $300 per gal).
Propiconazole is cheaper still on a per acre basis, but the label allows fewer applications and it doesn’t work as well as Indar.
Hence I am interested in tebuconazole (as jaypeedee mentioned) which may be another affordable option, as a replacement for one of these other azoles (DMI’s)