Coolant loss

My truck regularly overheats every 3-4 months or so. My truck always smells like coolant when I walk around its front, even when I have turned off the engine. I suspect there is a leak but I do not see a pool of anything let alone green coolant when it is parked. When it does overheat, I wait until it has cooled (2-3 hours), top it off with 50/50 antifreeze and the problem seems to go away for another 3-4 months. I have replaced & fixed a busted head gasket and a cracked cylinder in the past. Am I facing the same doom? What does this normally cost around the Monterey, CA area (since I have recently moved) and when or how do I suspect I'm getting screwed by a mechanic?