This question gets asked very frequently. However, there is no real definitive answer. It is very hard to quantify how much ink is used during a cleaning cycle due to slight variations from printer to printer and even ink pump to ink pump -- each printer is slightly different. It is safe to say though that the amount of ink used is minimal in comparison to normal printing and as such is used not taken into account when calculating ink cost per square foot.
On average, you can expect the smallest (economy) cleaning to take at least 2mL of ink; a normal clean will be about twice that (around 4mL) and a powerful will be around 3 times (6mL). Note that these are low estimates and the machine may draw more ink than that.
An Ink Fill typically draws 60mL of ink (per color) on most machines, but on the ValueJet 64" and larger machines, the machine will use over 110mL of ink due to the subtank systems. The ValueJet also includes a "LittleCharge" option which draws about 1/4 the amount of ink of a normal Ink Fill, or roughly 15mL before the subtank refill.