How can I check the height of an obstacle on a grid?

I created a plane with some obstacles on it. The obstacles are GameObject of type Cube with different sizes. (Different x, y and z). I also created a grid on top of the plane. Each obstacle can be on top of one or more tiles of the grid. Now, I want to iterate on every tile on the grid and see if that tile contains an obstacle, return the height of that obstacle(how tall that obstacle is?) I am implementing A* algorithm but I want to also add some height cost to it so that the pathfinder choose to either go on top of an obstacle or move past it on the ground(whichever is less costly). Is there a way to get the height of each obstacle on the plane(grid)?