Price Check on:

Very Large 16x Mega Rich Mining Toxic With Stealth Screen
Very Large Rich Mining Volcanic With Stealth Screen
Very Large Rich Research Gas With Stealth Screen
Average Extremely Abundant Volcanic With Stealth Screen

Basically, how much value does a Stealth Screen add to a Planet?

from 20% to 30% value added to the planet. You'll get much more if the planet is a wanted planet (VL icy, for instance)