The studs themselves run around $450 themselves. There is a number of gaskets and the like that should be replaced at that same time. Since you will already have the motor torn down when you do the job, do yourself a favor and have the oil cooler rebuilt and your EGR cooler solved (delete, blocked, or upgraded - I prefer the last option). You already have the monitor to determine when the oil cooler needs to be done, so I would use that to determine how much longer you want to go before doing the job.
Labor costs in your area is the biggest driver of the cost of this particular job. Around here its around $80-85, which for the approximate 30hrs job, thats $2400-2550 in labor alone. There is a huge savings to be had by doing this job yourself. Which isn't easy, but doable for the do-it-yourselfer.
As to the heads, the key things you are looking for is that they are tested for cracks, and checked for flatness. Even if you buy new heads, those will also need to be checked for cracks and warpage.
There are a lot of details to the job, outlined in the thread below:
Write Up: Bullet-Proofing the 6.0L PSD.
Take a look at your trucks MY flaws in that thread as well, and determine what else needs to be done while you are in there.