How much money does it cost to replace a head gasket on an 02 dodge ram?

It depends on who does the repair. It will be far more expensive at a Chrysler dealer than at an independent. Assuming you take it to an independent professional and if all that is needed is a new gasket and you're looking at probably $25 or $30 for the gasket plus $350-$450 labor. However if they head is in need of machining due to warpage or the head must be replaced because it is cracked you're looking at additional expenses. You will not know if the head is okay until it is removed from the vehicle. In addition to these expenses you need to find out why the engine blew the head gasket in the first place.