I can't help you on price to get it done as I've done it myself but I'll offer up some things to consider which will effect the mechanics costs.
First, I have changed just the belt before with only taking the intercooler piping off. It can be done but it's a real pain. I had to extend the handle of a ratchet several feet to reach down to the tentioner and relieve the tension. This was the most difficult part.
Here's the thing about 6.0 belts. They are very similar in function to a timing belt in an overhead valve engine. They last a long time. Also like a Japanese timing belt it is a very good idea to change all the other rotating parts at the same time. So for a 6.0L that would be all three idlers, the tensioner which will come with it's idler, and at least the vacuum pump because they seem to get weaker after about 80k. If your into preventative maintenance this would be the time to upgrade the water pump to say a Bulletproof Diesel metal one. This is also the time to change the power steering pump if you really have a lot of miles on it but maybe save it for next time. Also how old or weak is your alternator. Probably the easiest to change on it's own but it's staring right at you if you've done all the other stuff. But like the P/S pump it could go longer if your just not in the mood.
My recommendation is take one of two paths. If your just going to change the belt because your van doesn't have very many miles on it (less than 90k miles) but it's 10 years old and original. AKA the belt is old but the pulleys haven't spun much. That can be done without taking much apart. OR If your have say 80-90k miles on everything replace the items mentioned above as you will (or your mechanic will) be taking everything off between the A/C condenser and the front of the engine. Then, like the Honda with a new timing belt, be good for another 90k miles or 9 years.
When your getting estimates from mechanics to do this job be aware of what the mechanic is replacing as that will greatly effect the prices. Example, just the belt vs, and the idlers vs, and the other stuff.
Lastly, insist on name brand parts. Gates, Motocraft, Denso, ect. Don't let a mechanic use Dorman or no name parts. Parts are cheap compared to labor and tow trucks.
2005 SMB RB 4x4 6.0 PSD
A rocket on the pad is safe,
but it's not what rockets are built for.