Dirt, good question. Its seems to come in many forms, and most of it is like concrete. When it rains, you will be up to your ...... (well atleast) knees, and when it has dried for three days, your pick-axe will bounce off it.
It also comes in many colors, from white to red, and yes I have seen the dark brown (black) around, but no price on the latter, yet, I expect higher.
Seems to be sold by the cubic meter, small dump truck is 3 cubic meters and can cost between 350 - 700 baht. Depending on the purpose, its best to see it before you order. Watch out, you just might get a truck load of clay.
For a building site there is some red stuff which is the closes I could find to gravel, (again, watch out for the clay) with this stuff, it can rain for days and you can still walk and drive on it! But again watch out for the clay, walking on the clay will increase your height
Now the white fill was used across the soi from me, and the workers at my place, kept saying "no good". That was 8 months ago and the house has been finished for some time, and it looks ok to me, but give it a year and we will see.
Another place also across from me was filled about 2 months ago with a yellowish soil. I have no idea on it, not sure if something else will be added to the top. As of now no building or anything else going on.
good luck.