If you don't replace the broken stud, you will ruin the cylinder head due to fretting at the sealing surfaces.
You need to pull all the heads and replace the valve guides if they are original: thats why you see the excess carbon in some of those ports. You didn't say how many miles are on your car, but if its over 100K, you'll likely need new exhaust valves. The heads are all cleaned up during a proper valve job so thats not a problem for you.
Costs will depend who does the work and what parts are replaced,....
