Perhaps, Texgal will chime in. I have no experience with laterite, but you should see some of Texgal's aquairums where she used this laterite and capped with pea size gravel. Absolutely beautiful and to kill for. Iam not sure, but I believe Texgal mixed the laterite with peasize gravel and added that as a first layer and then she capped it with a thick bed of some more pea size gravel.
A Lfs owner setup a tank with laterite capped with Chicken/Turkey Grit(he cannot keep cory cat fish as it is too sharp for them). He used pressuriized c02 and a good lighting system, that show tank was really stunning.
Costwise though, you may be better off to go with fluorite or eco-complete as you would not save much using laterite to justify using it over a commercial substrate.