Using an Epson R3000 printer, I reckon about £15 (say US$23) per finished A3 print.
Seem a lot?
The reason I print my own is that:
(a) I want total control of my photographs from the moment I compose the image in the viewfinder until I mount the print.
(b) I am careless.
No matter that my monitors and printer are perfectly cross-calibrated, a print can never look exactly the same as a screen image. But, more than that, after examining a print, I will inevitably find a wee flaw that I didn't notice on the screen. Or on the first print.....or the second..... Or I just see something that I would like to make a little different.
The result is that I usually end up making 3 or 4 prints before I am completely satisfied. If I was ordering them from a commercial lab, I probably wouldn't do that.