You have to check firstly what LTE bands/Networks are available at your location. No point in buying a Truemove H package if there's no good signal at your location.
Cheapest 'max speed' unlimited data option is the NT Thunder SIM; 1 month, 3 month and 6 month durations for 199, 399 and 599 baht respectively. But unless you have good coverage of LTE Band 40, useless.
The way to find out if you have coverage is to go to Google Maps, find your exact location and then copy the Longitude and Latitude details from the URL and paste them into the NT Wireless website.
https://wcsoss.totwbs.com/registernew
Example of Udon Thani Airport: 17.387112 & 102.772953,17z
This is a fixed point wireless Internet service.