Rates at machine shops tend to run in the $100.00 / hr range for small work... any decent shop should have the right tools to mill the seat but most machinists of the old school would have spent a good deal of time doing hand work with files which is something the average person can also do with patience and attention to detail.

At our little shop you do get to deal with a couple of guys who were machinists before they turned their attention to building bicycles...