the price really depends where in the country you are. here the cost is usually a minimum of $600 and usually around $1000.
i plan on doing mine myself, but i have all the tools, i have complete instructions from alldata, and i will have guys helping me that have done them before.
you know how on the tv show trucks he has the level of difficulty scale. i would say gears are on the hard level. they are easy if you have done them before, but if you have never done them before i would highly suggest not taking on the project yourself.
also make sure they do everything, all bearings and seals as well. and consider getting studs and a cover with braces.