I have no idea about the average cost, but when I had dental (not including health), it was around $60-$65 a month, for me and two children. Sorry I couldn't help more...
I use health benefits. Not insurance, but close. I pay $20 a month for dental, vision, prescription, and chiropractic for everybody in my house. So I love it. The website is below. Hope that helps.
You will probably be better off with an affordable dental discount plan rather than dental insurance. See which plans are available in your area by keying in your zip code at http://www.lowpriceddentalplans.com