I've never heard of anyone reporting a successful integration of xF and D.
You'd have to get a custom addon made.
Cost .. no idea. I suspect other would be willing to help with costs as you aren't the first person to want xF-D.
If your stress levels are low ... and you want to increase them ... vBulletin might be a forum option. This is especially true if you want your forum software run by an unethical money-grubbing Mega Corporation. Can't say I'd do that.
Lost cost and not as good as Xenforo .. might be phpBB integration. http://drupal.org/project/phpbbforum
You would likely be able to migrate from phpBB3 to xF down the road if you wanted to pay for xenforo and an integration addon.
If you don't care about login integration ... you could just get xenforo and deploy it with Drupal ... but you'd need to probably Style xenforo to look like Drupal so the site.