i haven't had any real world experience with that printer, but I can tell you that Brother CORPORATE over estimates ink usage. They don't want to get sued, so they are very careful with the numbers.
As far as sales people go, I would be wary of most of the numbers they throw out. I can put you in touch with a guy I work with, he's a dealer, but when he brings samples by, he's very honest with ink usage and so on.