it's expensive and it's not expensive.
eg.. on bt, my 81 sf/3 scout/2 cart/5 healer/5 banner/3demo tools (no idea what cost what so listing it out...) is said to cost 90 gold and take 11 hours to procure and board ships.
now... 6 days' wages for that lot costs 166 gold. If sea travelling is far enough to save say... 3 or days' worth of travelling (that's including stopping at every small region) then it's a bargain. that doesn't even count the equipment damage.
that said.. have to take into account of re-provisioning as you'll have to land and get provision before embarking again.. if the journey is that long. totally forgot how much provision is eaten a day at sea. haven't tried using it to move troops yet. might do that the next time i refit.
