unfortunately "cost-effective" and Atlantis really don't go together. As far as i know to get '"full access" to all that Atlantis has to offer you must be a registerd guest. Used to be that if you booked into the Comfort Inn you could tour The Dig, use the beach but not the waterslides.
We haven't been there in a couple of years so things may have changed. If you just want to tour The Dig simply grab a cab- either water or land, you enter in by the casino there is a kiosk there where you can buy your ticket for The Dig - it is so worth the price! The Dig is awesome.