IIRC, there's no percentage-based commission, but Thai banks / currency exchange houses charge you a per-check fee, regardless of the value of the TC... in other words, if you're going to take TCs, be sure to take high value TCs (100s instead of 20s).

Like a poster above, I would also take a few hundred dollars in cash. Note that cash exchange rates are slightly worse than those for TCs, but there are no additional charges. High value bills (50s, 100s) get a better rate than those of lower value.

My usual practice is to change some money (usually cash) upon arrival at the airport (open at all hours, though with worse rates than in town), take a taxi in, and then change at the banks (many of which have currency exchange kiosks / walk-up windows) in town.

- Michael