I did a radiant barrier on my rafters to guide heat up from the soffits to the ridge vent and then insulated the joists. Blown in insulation is pretty cheap. While doing other work I also insulated my southern facing garage wall and of course insulated the garage door (and sealed the edges). This made a noticeable difference in the temperature of the garage. I can turn a couple of fans on in the garage and leave the door to the house open and achieve a very reasonable temperature in the garage if I plan on being in there for a length of time. This helps in both summer and winter.