Broken Variable Speed on Drill

I have a fairly basic variable speed cordless drill with adjustable torque and hammer action, with a 14V motor I think.
Recently the variable speed stopped working. All other functions are fine and the drill operates fine at max speed.
I understand that the linear potentiometer trigger mechanism which operates as a speed controller may be worn, but is this worth replacing?
I bought the drill for about 20, any ideas on how much a replacement part as above might cost?
I have also read that you can use an "inexpensive external motor speed controller" what is this and how inexpensive is "inexpensive"?
Many thanks,
Tanel.
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Is the switch worth replacing? You have to compare the cost of the part and the cost of installation (if you cannot do it yourself) to the cost of a replacement too..
Before going that route, though, if you do not mind opening the drill: try washing the switch with electrical contact cleaner (spray can). Dirt does many things.
Bruce Nelson

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