Most kits will give a recommendation of the size servo you should use. I usually ad about 30% or 50% to that and get a servo with that much torque.

Spend the money on good servos. Don't try to save $20 per servo and get cheap ones. If you had a plane with 6 servos and you saved $20 per servo on cheap ones- then you'd save $180. Big deal. How much is the model worth?

If you had a $2000 model and you saved $180 on servos and then one of them fails- it wasn't realy worth it; was it?