how is smoke test performed and how much $

The smoke test is normally used to find a vacumn leak in the emission system or air intake. It uses a smoke machine kinda like one used for parties and special effects but highly specialized for automotive testing. I.E. we would fill your gas tank up with lightly pressurized smoke and look for a visual leak that wouldn't be able to be found otherwise because of its location or size. The cost of this procedure is the same as any other diagnostic work in most cases. Some shops are proficient at it and will charge the standard 1 hour diag charge whereas some will charge you actual time, A-time can be to your favor or quite the opposite depending on how hard it is to find the problem and how good your mechanic is. If you need this testing done look for a master mechanic either with ASE or manufacturer certification. Although it may be a little more up front these are the technicians that will cost you less money down the road by repairing your vehicle properly the first time.