If you have spot colours, measure the square centimeters of each image, add them up and calculate them per spot colour.
For Type: estimate 20% of the area covered
for half tones: Estimate 50% of the area covered
for Solids: Estimate 100% of the area covered
If you are printing a 4 colour use...
Sometimes it is difficult to calculate the ink consumtion, I suggest to you to Google , Van Son Ink Estimation where you will find a cover page for yuo to calculate ink consumtion in relation to the kind of paper being used.