I am sorry to say that it will be very difficult to give a estimation of the cost you will have.
It all depends on the fact if just the potmeter is broken...or if also the PCB Board where the potmeters are connected upon is damaged.
A potmeter will cost just a couple of bucks....but if the board has to be repaired or swapped expect at least 100 or a couple more.
Here is a picture of the innards of the TI Desktop bottom...as you can see there is a big chance that the PCB board also got damaged :
