usually machine rates are charged per hour, rates can vary greatly from shop to shop depending on how important their time is or how expensive the machine(s) you will be tying up is
generally the more complex the parts are the longer the time required to manufacture them, therefore a higher price required
you will see price breaks at higher quantities but for one offs your essentially paying the most you can pay per part. We typically never do singles of anything (within reason;large/unique/expensive is a different story) but anytime someone comes in and says "I need one of this tiny simple part" we quote them for 5 minimum
I think the earliest factor that decides whether or not someones personal project actually happens is when they start getting quotes about how much it will cost.