For the record, my total bill was around $400. (Dashcam 56 - $140, Dashcam Mini - $90, 2 x hardwire kit - $60, Magnetic mount for Dashcam Mini - $15, Pro installation - $100.)
I got the magnetic mount for the Dashcam Mini so that I could easily remove the cameras and put them in my Flex, which my wife and I drive only occasionally. Before we take off in the Flex, we just pop the cameras out of our DD's and into it. Saves me having to buy a set of cameras that are rarely used, and our phones are already synced, so that makes it easy.
Lastly, the Garmin cameras come with a dual USB port cigarette lighter adapter and long enough USB cables in the box. This doesn't give you the Parking Mode option, but it's nice to have if you ever use a rental car. At least you have some protection.