It's a broad area. I've seen some shops offer to do the job for $199 for parts and labor. I know the dealer wants $300 alone for the timing belt and tensioner plus $50/hr for labor. So you can already see prices are all over the place.
I'm intending to do it myself here in a few weeks. Requires you remove the rad-support, balancer, belts and the timing belt cover from the motor. Than it's a all belt time from there. Some go as far as removing the front bumper, structural bumper and rad so they have much more room to work.
Been looking online for TB prices and you can have them from anywhere in the neighborhood of $30-$300, from Mopar to weird brands you'll never want to touch. Goodyear belt's go for about $70 which isn't bad.
Expect to be encouraged too do your water pump now as well, since you'll be right there to do it. Beyond that point, once you do a TB, WP and T-stat, your 3.5 should be ready for another million miles until the next interval. :P