I think that in this case embarking would cost a lot more than that. Iirc, the last time I tried doing it from an enemy rural region, it's cost was in the area of one hundred gold and would have taken over thirty hours. This was with around thirty cavalry, all wounded. After ditching the cavalry, I was able to embark within a few hours, at the price of only ten or twenty gold.
The high cost of embarking under such circumstances is why Aurvandil elected to spend several days taking over Saffalore, which served no other purpose than to reduce fees, rather than leave immediately after trouncing Terran. If the act made the difference of a mere fifteen gold per noble they wouldn't have bothered.
