The thread works pretty well. I did the same thing but used epoxy for the final finish. Be sure to seal the inside of your bore also. I think calls pick up most moisture from blowing hot breath through them. That plus a freezing thawing cycle while hunting will stress any wood that isn't sealed from the moisture.
Pricing, I would say sit down with a piece of wood and make a call start to finish. How cheap are you willing to work? At the least you should make the cost of materials back + a little.
I would rather give my call away than have someone tell me its worth $12.50.
Before you sell a call and never see it again, make a blueprint, take a picture, something so you remember it and could make it again. Then write down what you sold it for.