It depends on the monthly pageviews you're getting and where you plan to display the ads.
Last month, Mom's Daily Zen had over 6300 pageviews and over 4000 unique pageviews. Plus. my subscribers get updates right to their email inbox, so it is more effective readership in my opinion.
So, for the top banner ad (displayed above the fold) I charge $15 a month because the ad is displayed alone and at the very top of the page. Below the fold banner ads are $10 because they are still at the top of the page, just right below the fold. Button ads are a bit further down, so they are only $5.
However, if you plan to fill up your top columns with other widgets, you'll have to lower your prices to be appropriate for the ad placement.
You can take a look at how I have things organized to suit my desired pricing schedule at
http://www.momsdailyzen.com