Average point value of a defensive penalty: 0.914 points
Average point value of an offensive penalty: -0.229 points
Suggests that defensive penalties are more costly than offensive ones...
Average point value of a turnover: 3.17 points
Using the PBP, Rock M Nation calculated the number of points a team could expect to score based on its field position, and he used those averages to find the above values. More here.
Last year, I broke down the penalties committed during the 2006 season by NC State and ended up with these numbers:
Off 50 58.1
Def 25 29.1
SPTms 11 12.8
Plugging in those point values (which are derived from a different season, so this is definitely a rough estimate), the cost in terms of points looks like this:
Offense: 50 penalties x -.229 pts = -11.5 pts
Defense: 25 penalties x .914 pts = 22.9 pts
So 75 penalties made for a net cost of about 34 points in 2006.
As for turnovers...I'm not sure I really want to think about that.