maximillianos - 4:51 pm on Sep 6, 2010 (gmt 0)
There is no standard rate. It depends on your site/niche and most importantly your traffic/pageviews.
You can try it out with a number/price you are comfortable with and go from there. If after the first campaign they don't want to renew, you may be priced too high (or the ad just didn't match your visitors interests).
If they jump on renewing it, you may want to bump the price up slightly... etc.
I would also consider the potential loss of visitors you will incur by having a full page ad take over the homepage. Many folks will just hit the back button and move on to another site.