You're right: pennies per vehicle add up fast in high-volume models. But a grade sensor is not really necessary to give a modern car enough information to detect grades.
The engine controller can calculate the torque delivered to the drive wheels, based on airflow, ignition timing, etc. With a very simple model of the vehicle's aero drag and rolling resistance, and measurements like vehicle speed, an engine controller (or trans controller) can calculate a force balance on the vehicle. This force balance can be used to estimate the current road grade.
The controller doesn't need great accuracy in grade measurements to do useful things like downshifting on long grades, so moderate errors in the drag model are easily tolerated.
That said: software isn't quite free (memory and software development cost some money), but it's a lot cheaper and more reliable than a sensor in applications like this.
Geek note: the two most variable unknowns in the force balance equation are total vehicle mbutt (e.g., driver only vs full load) and road grade. To a very basic grade estimation algorithm, a heavy load might look like an uphill or downhill grade, depending on whether the vehicle is accelerating or decelerating, but with a little more work in the algorithm, a useful mbutt estimate can be created (with the buttumption that vehicle load doesn't change rapidly while the vehicle is moving). -- Chuck Tomlinson
