I believe Dropbox and Groupon were both quite expensive (but acquired after significant traction/funding).

Mint.com on the other hand was purchased with ~$180k in Series A stock. The domain owner ended up making $1+ million when the company was acquired. The founder (Aaron Patzer) seems to think it was one of the very best decisions he made.

The paradox of choosing a domain is that you buy it when your company is small but you live with it when your company is big.

It's a lot like choosing real-estate for a restaurant. You can launch your restaurant in a bad location, build it up, and then move. Or you can find a great location and build it there. A great location doesn't mean you will be successful (it's just a big advantage), and a bad location doesn't mean you will fail (it's just a big disadvantage).