"The barges, depending on the weather and any stops they make, usually take about six or seven days to reach Turtleback Ferry. I daresay they do sound more comfortable than horses, don't you think so?" The Lord-Mayor taps his stubby fingers lightly on the edge of the table, as if playing an invisible musical instrument, pursing his lips. "As for a spell of the nature that you speak of... given the distance, I'm afraid that even if you were to give us an instant appraisal of what is going on at Fort Rannick, we would hardly be able to send you any help to swiftly deal with a situation. Nor would it be very efficient - nor economical - to keep sending messages back and forth. I trust that you are aware that on a mission such as this, you would be acting as authorised deputies representing the Magnimarian Council? Perhaps some messenger pigeons would suffice to keep us here updated? Half a dozen ought to do the trick, I daresay."
Sense Motive DC 13
Grobaras seems (and understandably so) unwilling to part with any more money than he absolutely needs to without appearing stingy. If you want more money out of him, you're going to need to be a lot more persuasive...