I agree with everyone else that perhaps a road bike is not the best anniversary present. A romantic trip or weekend would be better.

That being said, you can still buy her a road bike with her help in your price range, but I would suggest getting something used. You can get something decent on Craigslist, but it will be at least five years old. If she decides to get serious you can always sell it and buy her a new one, keeping the old one as a trainer or backup. Triathletes are always upgrading their equipment and buying a newer bicycle, so lots of women are off-loading their old mounts.

Here are some example bicycles in my local Craigslist:

Cannondale R700 54 cm $680 "excellent condition"
Trek 1000 women 54 cm $650 (used only one month)
Specialized Dolce Women's 2005 $595 "barely used"

Good luck and have fun. Happy anniversary.