They don't let poor people sponsor other poor people to come and live in Thailand. Pretty sad salary from an international school if you don't have 40k THB saved by now but you can fix that by doing what was posted above (cash withdraw, deposit, DON'T drink it all up).
The letter is just "Dear IO. I Joe Smo, holder of Brit PP # xxxxxxxxxx, Non-B Visa # xxxxxxxx would like to sponsor my wife Jane Doeski, Holder of Polish PP # xxxxxxxx to come join me in Thailand at my home Address: 123 Soi 4, Nakon Nowhere, Thailand, xxxxx, Thank you, Joe Smo"