If I remember correctly, sodium thiosulfate only works to neutralize chlorine, not chloramine, which is being used by more and more water companies all the time. Chloramine is a mix of chlorine and ammonia, so just neutralizing the chlorine isn't enough. I'm not sure what is in Seachem's Prime.
I use Prime from Seachem. It removes chlorine and chloramine. It detoxifies ammonia, nitrite, and nitrate. It also detoxifies other heavy metals and adds a slime coat to fish. A 0.5mL treats 5 gallons. So a 250mL bottle is good for 25,000gallons. Not bad for the price....not sure why you want to play around with your fish's well being to save a few bucks. Its one less thing to think about in the back of your mind if something does go wrong with your fish.