I change my own as well.
Its not $0, or "Free". How much did the changing equipment cost? Even if you are like me and made your own DIY changer out of an old car rim, some 2x4s and misc. hardware, the tire irons cost some. Also your time isn't free. So if it even only takes you an hour to change tires (more like 3 or 4 for me), do you work for minimum wage or more than that? Even if you discount your labor costs, there is still the opportunity costs, you could be doing something else instead of wrestling rubber.
That is the actual "cost" of DIY tire changing.Most people will find that its actually cheaper to have a shop do it unless you have more time than money or you a masochist who likes doing it (me).