Go with dirt mix. You can add perlite(buy it in cubic yards bags at Ace or landscaping) and add it to topsoil and sand to get a good mix. Once you get that right, you can even sometimes add in earthworms to help keep it dug, and add good bacterias. The lizards will enjoy digging and burrowing much more in real dirt instead of mulch too, since they can't make as good of a burrow in mulch.

As far as how much dirt? It takes a LOT. In a 3x6 ft enclosure for mine, I put a good 400 lbs total in without it looking all that full. PLan on raking it around with a 3-prong digging tool often, if needed.