It's pretty simple actually, if he plays 100% of games then let's say he's worth $1m. Since on average for the last 2 years he only plays 1/2 of the games then he's worth, say, $500k. If he plays more than his average number of games then we get good value for money, if he doesn't then we get bad value for money. Same as the employee you would like to have for 5 days a week but who only works 2.5 days a week.
The cap is very limiting is cases like this, hence thinking outside the box is required. As previously posted, the Chooks did it with Cronk and Melbourne with Slater.
Cheers
Gary