Hmmm, At those dosages and using the largest bottles(on HFF's website) assuming those are the cheapest per volume...
Seachem Prime 250ml bottle costs $34.30 to treat 10,000 Liters making it .34 cents per liter
API Stresscoat 1.89L costs $57.50 to treat 15,140 Liters making it .38 cents per liter.
So, pretty much the same price but stresscoat claims to be good for the whole replacing the slime coat and stress thing.