It depends what location you are at. Most of the dealerships buy the shirts from outside vendors at cost, and will mark them at a price that suits the dealership's sales.
Men's shirts usually start at about $22 and can go up to $35 or so for T-shirts.
Women's shirts usually start aroun $19 and can go up to about $28 or so.
Varies a lot. At a typical dealer, not in some tourist area, on the low side around $10-$12 bucks, on the fancier-looking ones more like $19-$24. In a tourist area, they will be around twice that range.