Here is 1 kg of seachem safe that treats 200,000 gallons (yes 200,000) for $29. The site is legit too. Prime cannot compare to safe as far as bang for buck goes, as a comparison 2 liters of prime is $50 and treats 20,000 gallons. And again safe is the powder form of prime, smells horrible though:
http://www.petblvd.com/cgi-bin/pb/JRP03870.html
http://www.petblvd.com/cgi-bin/pb/JRP03870.html