You'll likely need to remove the manifold [and turbo, dump, water lines, oil lines, intake pipe etc] about two hours work. Then you have to remove the manifold. Use heaps of CRC/WD40 on all the nuts and studs.
Once you have the manifold off [and you could snap a couple more studs doing it yet], see if you can get vise grips etc on the protruding part of the stud and reomve it. If it is too short to do this, then you can try welding a nut onto the end of the stud and then remove it while still warm just like a bolt. If the stud is snapped off at the block then you have to be very careful and drill a hole in the stud and then use an ezi out [this is a mongrel act, plenty of cursing and skun knuckles, but be very careful and patient]. Hopefully all this will work, if you snick the thread while your drilling, you may need to use a tap to clean up the thread before you replace the stud. When replacing the studs {I'd replace all the manifold studs, buy a kit], nuts, and bolts etc use nickel or copper cote antiseize compound. Use a new stud and gasket set/s for all the job. Also get the good studs, there are cheap ones out there that you should avoid.
Cost, around $150-$200.
Too easy isn't it!!!!
Or you could take it to a shop and they'll do the job for around $500 or more.