It's kind of hard to put a simple number on that (of course, you could have done tons of research to support this, I'm not sure). But it turns out one of the ordinary byproducts of ordinary rockets is water, which is why all space food is dehydrated. This means that if you sent a rocket with nothing on it at all (zero payload) it would arrive at the moon with water.
Of course I don't know how much and it is still likely to be ridiculously expensive.
Ok, so I'm a little sketchy on where I heard it originally, bu I think I was watching some sort of space show that said that hydrogen and oxygen mix together to propel a rocket but they capture some of the emitted water vapor and turn it back into water for other uses. I can't find specific reference to this recycling on the internet, but liquid hydrogen is a commonly used rocket propellant.
Of course I don't know how much and it is still likely to be ridiculously expensive.