Well, I just got a couple more quotes and $500 - $600 seems to be fairly reasonable. It is not an easy job removing everything from the manifold etc... Then if any of the studs are stuck, it is a little tricky drilling them out. Machining of the manifold is about $100, studs are about $12 including nuts from Nissan and exhaust manifold gasket is $115 from Nissan. I've found no general mechanics want to do it, only exhaust centres, so it is obviously not the most enjoyable job to do.