Everyone seems to think it's about maximizing his ROI or, less cynical, about economics. But here's a quote from Thiel's speech at the republican convention:
"Instead of going to Mars, we have invaded the Middle East. We don't need to see Hillary Clinton's deleted emails: her incompetence is in plain sight. She pushed for a war in Libya, and today it's a training ground for ISIS. On this most important issue, Donald Trump is right. It's time to end the era of stupid wars and rebuild our country."
He clearly says "On this most important issue, Donald Trump is right" and it's not really surprising, since Thiel is a libertarian.