You may want to look up Seachem's "Safe" product. A 250 gm bottle goes for $ 8.99 and will condition 50,000 gallons. I get it from CustomAquatics which is one of the links provided in one of the posts here. It is the dry version of Prime.
I dilute a 250 gm bottle with two liters of distilled water and use it the same as the dosage for Prime. A two liter bottle of Prime will cost you over $ 30, even at Big Al's.