There is no "activation" cost in Thailand, just buy a sim, insert and call.
SIMs are dirt cheap, usually 50 baht, with 50 baht in call time credit, so basically free. Calls are anywhere from 50 satang to 1 baht a minute domestically, 1 to 10 baht a minute internationally. There are a myriad of plans offered with prepay rates.
All three cellphone companies have good coverage in Chiang Mai, AIS, DTAC and True. AIS has the best rural coverage, True has the best international calling plans. All three offer prepaid data packages.
All three companies have info on their websites on rates and packages in English (though sometimes you to keep selecting EN at the top as the sites don't always remember your preference from page to page).