Here's the math, do correct me if I'm wrong. All prices in CDN$.

For cal hypo, I bought 25 kg of 70% cal hypo for $77.93, that's $0.0031/g, you need 29.5 grams for 1 ppm in 20000 liters so that's $0.09/ppm. (Source: Taylor Chemistry Guide from my K2005)

For the Canadian Tire Purox, it's $4.99 for 5 liters, that's $0.998/liter, you need 0.2 liters of the stuff for 1 ppm in 20000 liters (assume it's 10%) so that's $0.20/ppm. (Source: www.canadiantire.ca)

For the IGA Javex, it's $2.49 for 3.6 liters, that's $0.69/liter, you need 0.4 liters of the stuff for 1 ppm in 20000 liters (assume it's 5%) so that's $0.27/ppm. (Source: www.iga.net)

I think the math works out, let me know if it doesn't.