I trained to be a D.I. about 10 years ago and it cost over £2000 then, spent about 6 months working as D.I. whilst training, also doing the windows at same time.Gave it up as not earning enough, I was given an area to cover that was 20 miles from where I lived, so petrol bill was enormous.
Worst bit was trying to teach people to drive in the dark, especially country lanes.