I haven't had to do this to a manifold bolt yet, but typically you drill out the old bolt and tap a new thread to a larger size and either use a larger bolt or install Heli-coils to restore the threads to the original size. (Heli-coil inserts are available at your local hardware store.) How easy/difficult this is usually depends on your experience and how much working access you have to the broken part.
I like to begin the drilling part by using a high-speed Dremel tool with a diamond grit tip to start an alignment hole that is centered properly in the old bold material. Then I use a FRESH, HIGH-QUALITY (cobalt) drill bit to do the drilling. I have a cable extension for my drill that helps drill in limited locations when needed. Once drilled, tapping a new thread is usually pretty straight forward, although again, the ease of doing this is access and experience dependent.
The cost of doing this depends on how many of these tools you already own. If this all sounds like Greek to you, you might want to have an experienced buddy help you out.
CraigK
