I have rethreaded crank arms and never had a issue. I have tap kit that have various size taps in it, but you could buy a single tap for around 10.00 its not hard to do. My guess a LBS or machines shop could do it for you, Some of the guys here who work at bike shop could tell you but I would not think they would charge more than $10-15. That would be the most cost efficient unless you found some freebe crank arms.