It depends on if the magnets did any damage to the other part of the sensor that is mounted in the engine. Just the top part of the cam sensor can be replaced for about $35-40 plus labor. If the synchronizer housing has been damaged by the loose magnets, it will add about $185 to the job. The labor to replace the cam sensor is about 1/2 hour. To do the synchronizer will add about an hour. Some books will say 2.1 hours but that is excessive in my experience. I have done 5-6 in the last 12 months.
I hope this is a help.
I had mine done yesterday.
Fortunately I did it myself.
Parts cost me $53. (RockAuto.com) Dorman (chinese) Synchronizer, sensor and alignment tool all included. If you want to buy the better quallity parts from the same place will cost $106 MOTORCRAFT brand.
My mechanic told me it will be $180 for parts and $120 for labor.
So i did it myself. It took me 2 hours for everything.
Hope this will help.