By chance I do have it all connected up right here: a Pi B+ and a touchscreen with a seperate power supply on an (albeit cheap) power meter. The meter is connected on the line side of a switching power supply, so the current calculated for 5V is really lower than these numbers:
Display powered, but connected to a switched-off Pi: 0W (cheap meter, I said...)
Display running, displaying all black: 3.2W (~640mA@5V)
Display running, displaying all white: 3.0W (~600mA@5V)
Display running, backlight switched off, black image: 0.8W (~160mA)
Display running, backlight switched off, white image: 0.6W (~120mA)
edit: When the console screensaver kicks in on latest raspbian, the backlight is switched off, which is good. But the screen is also "blanked to black", which draws unnecessary current. I know that the linux console screensaver blackens the screen since the invention of the nipkow disk, but it would be great if this could be changed to white here...