An inch of ink is just an inch of ink but it depends what happens within that inch.
A cover from crapcovers.cc aside from other problems may be at 200dpi, compared to a 300dpi image that is 33% LESS information so what are you missing? colour variances, fine detail and sharpness which is why 300dpi is the standard for the print industry.
Other factors like print settings will play a big part in ink usage and how can you tell exactly what the 2 of you are printing? Do you do other print work apart from covers?