I agree that the most likely cause is the cooling system's thermostat. This is one of the cheapest engine repairs that exists, so you won't need to take out a second mortgage in order to pay for it. The exact cost will depend on how difficult it is to access the thermostat, but rest assured that the cost will not be high.
In addition to the problem of inadequate heat from the car's HVAC system, running a car with a thermostat that is stuck in the open position results in a fairly significant drop in gas mileage and it also "wears out" your motor oil faster and increases wear on the engine. I would suggest having this job done sooner, rather than later.