Well, I'm no expert on these by any stretch of the imagination, but I do have a kit on order and have been doing some research on them.
The machining needs to be done to the bolt, barrel extension, lock frame and accelerator. The cost varies from $60 to $125, depending on who you have do the work.
The side plate comes already machined. The 100% is ready to rivet on, if you go with a 80% you have to drill and chamfer the rivet holes.
Lots of good info here. http://www.1919a4.com/