From my own extensive Thailand experience I know this happened rarely. Not only Western foreigners are targets of dual pricing methods but sometimes Thai people too. Dual pricing is officially not allowed.

If you are not willing to accept dual pricing methods, you can complain about this practice here:

The Office of the Consumer Protection Board in Bangkok (Hotline: 1166; or Tel: 02-6297048-50 or 02-6297061-3, from 8:30 am to 4:30 pm Monday to Friday). Email: consummer@ocpb.go.th; website: www.ocpb.go.th

Some English speaking staff is available. It makes sense to use easy and polite words. Best is to report your complaint in detailed writing.

Be aware that some claims are difficult to investigate if the customer was notified of the fee in advance.

There are no price control regulations governing medical practitioners.

Please note:
1. There are also no control regulations regarding "off fees", massage services or payments to moneyboys
2. There are also no control regulations regarding "special offers", "happy hours" and others