I took the last 20 years of player salaries and adjusted them to 2008 values; I did this by taking the average salary for each year and calculating the ratio between that and the 2008 average.  I then multiplied the salaries by the ratio of the 2008 average to their average.

While the average player's peak in terms of performance may be around 26-30, their peak in terms of getting paid is 33 to 37.  Breaking the players down into three piles of about 6000 player-seasons each:

18-26 $998K

27-30 $3.2M

31-50 $5.1M

Using OBP*TB as a quick and dirty measure of run value [and assuming that the hitters and pitchers pay distribution was similar], the old players [31+] provided, slightly less value than the middle group, yet cost over 50% more.   The young group provided 85-90% of the value of the other groups, but for 20-35% of the cost.

I'm pretty sure I knew that young players were a bargain [free agency rules and arbitration process pretty much ensure this], but I didn't think the other side of 30 was THAT bad of a bargain.