I agree with Chuck (Two Tracks) 10% is a good guideline, sometimes a bit more, sometimes less.
From a pure dollars and cents standpoint, if the bow is in excellent condition and you can sell it as is, and replace it with a similar bow of the weight you want, it would be cheaper that way. But as Chuck said, other work can be done at the same time, so if the bow needs an update anyway weight reduction makes more sense. Also of course, if it is a bow that has some personal history or attachment, you might really consider a weight reduction and get it back to the point where you can shoot it again.