Check this thread out and you can get your build sheet and see what gears you have in your truck. You will actually get your build sheet as to how your truck rolled off of the line. How Is My Truck Equipped?
If you are wanting to get bigger tires I think you would want at least 3.92's but if you have to change gears think about going 4.10's. As for cost I am not sure it is something I have thought about but haven't done a ton of research on it as I don't have the money to do it but it will depend on what manufacturer of the gears you are going to go with and then what the shop will charge you.