LPG costs from around 12.XX baht upto 14.5 baht in the sticks.

Before you go and commit yourself to Fofos, try this company out.

Linky - Energy Reform ผู้นำด้านอุปกรณ์ติดตั้งแก๊สรถยนต์ LPG และ NGV, ติดแก๊สหัวฉีด Advanced OBD จากประเทศอิตาลี.

They are having a very aggressive promotional campaign and costs are around the same as above.
I got a standard tank (70 odd litre) put in the other month and the missus didn't like it (even though we had the same one in our previous car!) and the dudes at Energy-Reform were happy to strip it out and replace it for a donut tank for only 1,500 baht extra.