Yeah, we live on an ancient flood plain, so we don't have "dirt" as I expect you know it. It's basically beach sand, so it may not be a good idea.
But, honestly, if you're going to that expense, rainwater tanks might be a better option.

Ah, I see. That does seem like a cowboy job if you've got a deck above a roof.
If I understand correctly, it might be a better idea to put in a waterproof membrane or something before you finish the deck.

Good luck!