Antillia, somewhere to the west/southwest of the British Isles. How much would it cost to travel there? I'll use the prices in the 1981 D&D Expert book to guide the calculations.
In my opinion, the first thing the party should do is hire a captain. In my experience most players want to buy the ship first, but how the heck would their PCs know whether they're getting a good vessel or a lemon? Starting with the captain gives you expert advice on every other aspect of the problem. The normal cost of a captain is 250 gp/month. He'll work for the PCs at least one month before the actual expedition.
Since the only navigational aid in the party's possession is some musty scroll probably found in a stinky old dungeon, the captain urges the party to hire a navigator right away. "You don't want the navigator plotting the course as we leave the harbor, after all." The navigator earns 150gp per month and will also work for at least one month prior to setting sail.
The captain and navigator study the scroll and estimate it will take a week to get there. The party decides that this initial voyage will be a short one: a week there, two weeks of scouting out the island, and a week back. So they will only hire everyone else in the crew for one month. So the captain and navigator will end up costing a total of 800gp.
Sailor's earn 10gp per month and at least 10 are required to operate a small sailing ship. One of the more grizzled players urges the party to hire 15 sailors. He fears the DM will kill some of the sailors and then pimp over the party somehow when the ship is understaffed. That'll be 150gp for the crew.
So the party has spent 6,950gp and maybe thinks they're ready to go. The DM smiles and reminds them they have to feed everyone for a month. Including the PCs and their henchmen, that's approximately 25 mouths. We'll round up to 28 people for a consumption rate of 4 week's rations per day. For the first week everyone could eat standard rations for 20gp a day, or 140gp. The remaining 3 weeks will cost 60gp a day for sea biscuit and beef jerky (iron rations), or 1,260gp. That's 1,400gp for food.
Drink is a little trickier to calculate. How much hydration does one wineskin provide? I don't really know. PCs in my game tend to guzzle the whole damn thing when adventuring gets rough. So maybe I'll just arbitrarily decide the expedition needs 10 barrels of wine at 100gp a piece, or 1,000gp total.
Some of the players will probably complain on spending 2,400gp for food and drink, but them's the breaks. If they're smart, they'll round up to 3,000gp as cushion for windless seas and other emergencies leading to a longer expedition than planned. They'll also bring 600gp in extra food and drink for each time they plan on carousing while on this little voyage.
So here's the final tally for our hypothetical sea voyage:
6,000gp vessel + boat
3,000gp food & drink
Note that's a small boat, no mercenaries to guard the ship while the PCs are playing away team, and no money spent on miscellaneous equipment the party might need once they make landfall. And only a two week window to find anything interesting (i.e. loot to make the voyage profitable).
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