Last year I paid 280-350 baht per 3m2 small truckload (couldn't get a large truck down the lane to my land), this was for general agricultural rice paddy land fill, no rocks or rubble in it. The material varied from clay chunks with an alluvial sand matrix to something approaching topsoil, which was darker and contained no clay. I'm 26kms out of Chiang Mai, so your quote of 800 baht for 10m2 is pretty good considering your location.
Being a bit lazy, I haven't read everyone's posts yet, so at the risk of looking like an idiot, I'll just blurt this out even if it has already been mentioned, that it's important to know which part of the land you are going to be building your structures on and make sure the crappy land fill material goes there.. any areas where you know you will be planting trees/crops etc, reserve the better soil (if available) for those areas, even if you will be using raised beds and adding in more soil at a later date.
My neighbour made the mistake of not specifying this, as he was away when his land was raised, and now he has a real struggle on his hands getting things to grow in/on the clay and boulders that is his garden.