lol well my 10 gallon containers must be alot larger than 10 gallons, because 1 bale just barely allows me to transplant 6-3gallon bags to 6 10 gallon bags.
also my "35 gallon" container takes almost two bales.
my 10 gallon bags are 13" in diameter by... i think like 1 foot. id always assumed they were approx 1 cubic foot. ill have to check their dimensions again.
However, is is just not true that its the cheapest. HP promix is at most 25% perlite?
go to any major garden sale, you can buy a compressed 4 cubic foot bale of peat for 20ish bucks depending on where you live. down south here, good quality spagnum was 26 bucks. add a cubic foot of perlite... or if you are going by expanded volume, 1.75ish cubic feet...
4 cubic foot bags of perlite are like 22 bucks... so your total cost is going to be about mid 30s...
i get you guys like to just bust open a bale and go to town, but saying its the cheapest is not correct.