Roughly $80 to $270 range for those brands available here.
Non pro version - cheaper and cannot control the exact humidity level. Agar agar setting - depending on the weather and your rate of opening the cabinet, the level fluctuates over a small range. For camera, it is not a problem.
Pro version - can control to the exact level. Not necessary for cameras to be exact but psychologically pleasing if you have another $100 to throw.