Have conducted a test on the actual current consumption of Pi with Touch Screen of the 7" official version connected to DSI and Touch connections via I2C.
The installed OS is Raspbian Stretch Lite (4.14.56-v7+) and for this test the display shows a white background with "Hello World" in the centre. The Pi (3 Model B Rev 1.2) was connected to my network via WiFi and I was logged in from a remote ssh terminal to conduct the tests.
Power consumption is as follows:
Pi only, display power disconnected: 260mA
Pi + Display with varying levels of brightness:
0 = 325mA
10 = 325mA
25 = 345mA
50 = 380mA
75 = 420mA
100 = 460mA
125 = 510mA
150 = 590mA
160 = 635mA
175 = 730mA
200 = 765mA
225 = 740mA
255 = 680mA
The brightness levels were controlled with "echo 100 > /sys/class/backlight/rpi_backlight/brightness"
The highest brightness off the display occurs with a brightness setting of 200, further increases cause the display to go dimmer.
Deducting the Pi's base power consumption, I would say the display draws between 65mA and 505mA, depending on the brightness setting.
I was also curious to see if complete blanking of the display reduced the power consumption so I used "echo 1 > /sys/class/backlight/rpi_backlight/bl_power" to switch the display off in the same way as most screen savers, but the power consumption remained at 325mA, same as when the display brightness is set to 0.