Ken's sells Prime by the gallon and is cheaper then anywhere else -
http://www.kensfish.com/waterconditioners.html
Pssst, Trick of the trade, Pond Prime is EXACTLY the same as Aquarium Prime...if you can find that cheaper in bulk, buy that.

However go by aquarium dosing instructions.
I've seen Big Als take their Prime down to about $60-65 before, and if you price match them to Ken's they give an extra 5% off. Could probably price match it anyway. I tend to stick with just buying from Ken though, great guy.
http://www.bigalsonline.com/BigAlsUS...hemprime4liter
As to the OP, why switch? Is there a underlying reason to? Don't change it if it ain't broke IMO.