Well you said mill, that implies metal. Do you need to mill metal or can you route wood/plastics? There is a big difference in money there, most cnc mills for hire are machinist shops that have to charge you a fortune due to their high overhead.
CNC routers on the other hand are more affordable, more hobbyists and prosumers have them and are willing to use them for you a lot cheaper.
Same goes for what Evan said though, you can get cnc routers cheap on ebay.