Okay, I guess I just don't get it. I came out way (way) in the black the first year I gardened, and I have every year since.
Buy seeds at the dollar store. Once. Save them for the rest of the years. It's okay to make an exception for tomatoes.
Double dig if you can. Scavenge for soil amendments, or use pee. Water with greywater as much as you can. Mechanical pest control.
Then eat.
By greywater do you mean a bucket in the shower while the water heats up? Using actual greywater (from sink/shower/laundry) is a big no-no for vegetable gardens.
I had bath water in mind, but we only use Dr. Bronners if the water's going on vegetables, and my husband takes a lot of baths. You're right that the shower bucket doesn't yield as much.
But water for us isn't a big expense, so I've probably not fully thought it through. If you plant intensively and mulch, it doesn't take a lot of water, but setting up to make that work is very labor-intensive.
We're plumbed for actual grey water here, as in the pipes are in place for it, but haven't got the system going because I haven't yet sorted the right way to change my water (soap) use habits to make it safe, plus we don't need that much here as I'm just getting the gardens restarted.
If nothing else, though, if we get another drought like the one in 07, we can save some trees that may already be stressed from the house construction.
So, I'll maintain that it's possible to mitigate watering costs with things like the bucket method you mention, but I can see, now, that water is likely to be more of an unavoidable expense in other people's circumstances.