1998 Honda Accord Steering wheels slightly vibrate at idle

I noticed a vibration with my steering wheels while driving on highway. Took it to the shop for wheel balance but the problem still remain. The technician at TirePlus tell me that the reason may be one of the mounts went bad.

For now, the vibration (just in the steering wheel) is not too bad but noticeable every time the engine sitting idle (even in gear or not) or driving at 65+mph. Any ideas on what cause the problem and how much to fix it?