I've had a bottle of starsan now for nay on 15 years and it's still 2 thirds full. You only need a couple of drops in a spray bottle and it's no rins. I think it's less than 5ml in a whole fermentation bucked and once it was in the to clean the bucket I threw the bottles the hoses the lot in and then started at scratch.
Brew co do it for 13 quid. It'll last you for years and it's food grade. I used to use thin bleach on a stubborn barrel but I wouldn't want that in my ale.
If anyone's desperate you can have half of mine.