Farm land is still pretty inexpensive but the price goes up when you factor in access to electric, water, roads and how close you are to town. As of now only Thais can own land. Generally if they know that a faring is involved the price doubles as they are aware of the disparity in revenue. You may be able to go to the land office and find out what land has been sold that is comparable to what is offered. If you find a friendly person there that is helpful. 400000 baht is about $12000 and there are about 2.5 rai per acre.
There are also 4 types of chanote, I would familiarize my self with the ramifications that are inherent in each and the color of the emblem. Many of the titles are rights to work the land. I believe only one is outright ownership.
The titles we have are restricted to the families of the original people that worked the land and can only be sold to relatives, but things may have changed. Have an attorney check the chanote before money exchanges hands or a cousin may come and claim title or a bank may have a lien on it.