A point not mentioned often times is farming out the work also farms out the liabilty of screwing up. If your company buys a mill and you make a mistake while using it, your company will pay you twice to make one part. If the outside vendor errs, your company still only pays once.
Tools, tool holders, and work holding can accumulate in a hurry also. I have 45 tool holders originally purchased for average $100 each including retention knobs. I also have 7 vises which were $350 each. This does not include actual cutting tools. Then there's the coolant and way lube. The related compressed air plumbing and wiring. So, the related stuff to make a machine usable is not insignificant.