Build a database and have a flood of

cashed-up customers on-demand

How I would do it to get maximum result for minimum cost

What I would do is set up a promotion and send it out by email first for free.

Then for anyone who didn't have an email address, or who simply prefers to get stuff by normal mail, I'd send them the same promotion by normal mail.

So I cut down on costs by doing what I can the free way first (email).

Then I'd pay the postage and printing fees to get my message to the rest of the database, and make sure I cover all bases and market my promotion to everyone on the database by sending out the promotion by normal mail later.