Yup. They (the farmers?) do sometimes sell to house builders but more likely the farmers will change to growing sugar cane which is worth more to them.
Looking at the figures above of 500kg/rai lets say a farmer has 24 rai. he would get about 16 Baht/kg from the local mill for his years effort so with 24 rai that would be 192,000 Baht less harvesting costs of 600 baht/rai (14,400 baht) less fertilizer cost of say 3,000 baht less other incidental costs say 1,000 baht total value to the farmer would be 192,000 - 18,400 = 173,600 baht. That is his years income. It also assumes top money from the mill at 16 baht/kg. Quite often this is a lot less at anything down to 12 baht/kg.
Growing more exotic varieties will not necessarily make more money for the farmer and the local mill may very well refuse to have anything to do with anything other than Hom Mali (jasmine) rice.