FYI, Mg chloride from BRS is $76.99 if you're a preferred customer (or if you buy 3). So 3 of these plus shipping to my address is about $350 taxes in. I think the 5-gallon pails are 30 lbs (13.6 kg) so this 90 lbs (40 kg) would last you 10 weeks at your current uptake which comes in at around $1800 per year. I'm not sure you could find it any cheaper. Eventually with growth, you will be looking at doubling that amout if you're doubling your volume to 1000 gallons.
I hope you're a heck of a lot richer than me.
FM Magnesium chloride when buying 4kg pails would be like $2500 per year not considering any shipping charges. I'm not sure if FM offers larger sizes as custom orders or not. The trouble is that these chems are heavy and cost a fortune to ship.