I don't have them in my truck but I have inquired about the cost. On average here in my area of Tennessee it'll run you about $150 per seat. I know this because I was wanting leather added to my truck and to add the heating elements was an additional cost.
Keep in mind this includes the labor and parts for it. You should or may have a blank (empty spot where a button should be depending on your option package) near your auxiliary outputs but they'll have to use the switches that come in the kit in those blanks.
Your dealer or almost all dealers allways outsource this type of work and it's cheaper because the dealer gives them a large volume of business. If you want to really see how much you can save by going through the dealer, ask one of the salesman who they use to do that type of work and then call that business yourself and see how much they'll charge you if you just show up one day to have it done. They will charge you about $150 more than they would charge the dealer for the same work, trust me.