Costco is offering a $5.00 manufacturing savings off a twin pack of (0.75 gal container) of Wet & Forget which works out to $0.10/oz. The regular in-store price is $24.99. Or you can buy it online for $26.99 [costco.com]
with free shipping. For comparison Home Depot sells a 1 gallon container for $30.8 $ ($0.24/oz) and Walmart sells it for $28.71 ($0.22/oz).
This is a concentrate cleaner that is mixed with water (1:5 ratio) and I use it for removing the algae on my house siding.