20% of 500k is 100,000 THB. Plus the initial 10k. Plus border run fees, plus another 10k for in-country extension, plus tax for anything earned over 150,000THB (for the 2nd 180 days of the year).
Could be 120k a year plus income tax (worse case scenario).
One of the teachers at my international school was on a Non-O visa and was able to get a work permit easily. The only problem was he had to leave the country every 90 days whereas with a Non-B you can do the 90 day reporting in-country. This caused a bit of disruption in his classes, maybe this is what the school means?
an agent can save you 4 flights to Bangkok or Chiang Mai and back. If you live near VFS (Bkk office or CM office), go yourself in person. If you live far away, it's much cheaper and easier to use an agent who can send and collect the passport for you. I used Mark Jacob this time and I live on Samui.
If you're in Chiang Mai, I'd suggest a visa run to Laos with a company. I hear the embassys at Savannakhet and Vientiane are easy to get visas and easy to renter Thailand when you have a history.