A dowry is an antiquated (sp) con that Thai families still try and pull under the guise of "culture" to see if the farang will fall for it. Most guys do fall for it still.
Also, it is an another angle where the foreigner has to abandon 100% of his culture and accept 100% of the Thai culture. It should be a compromise somewhere in the middle. A dowry is no compromise, it is a scam.
Don't pay it. Just say it is an insult from your culture to pay for a wife. Plus you will be taking care of her for the rest of your life (no matter what job she has, you are still expected to support her AND her family) so the family should be paying YOU! I know they won't, just being sarcastic.
If you tell her no, and she leaves you, then better to learn now that she is after your money than later. And it is about money, the whole saving face is just an excuse. They will get over the whole face thing in due time.