San Juan del Sur, Nicaragua
fits everything you want, there's a few surf camps around if you want to stay in such places otherwise you can find places as low as $500 a month, small town, lots of expats, good internet, good surf, night life prices won't be as cheap as rest of nicaragua but with $1500 way more than enough, you'll find teacher easily, i was there a few weeks ago, very nicely developed
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Corn Island, Nicaragua
there are two little islands in the atlantic coast of nicaragua, big corn island and little corn island, way less tourist than sjds, about $100 flight from managua, pretty remote and isolated, internet is not the best but you will find it, only thing here is no surfing, surfing is bad in the caribbean coast, the isolation is worth it though imo, you could find a teacher but everybody speaks english here in the caribbean coast it's the afro side of the country, i have been here a couple of times a few years back
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Roatan, Honduras
this is an island in the carribbean coast of honduras, generally i wouldn't recommend anyone to go to honduras right now but roatan is a bit different, it's more protected with lots of ex pats and million dollar houses here, good internet, way more developed than sjds and about the sames prices, the island is huge compare to the corn islands, no surfing as well, i was here about a year ago
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around Poneloya, Nicaragua
northern part of nicaragua's pacific coast have a few surf camps, from what i hear the surf is great but these are less developed areas, internet might not be the best or not at all, should be way cheaper than sjds, i think mcbaseball visited this beach, i was there a few years back, don't know much this region
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don't know much about costa rica's beaches, been to a couple but not that familiar i mainly go to san jose. i wouldn't recommend el salvador, and don't know guatemala that well. there's also the option of panama city, would be a different experience but i love it down there. i could easily live in roatan the rest of my life. i think sjds has the best mix of prices, isolation, amenities for your trip and in a pretty safe place, for the most isolation the corn islands.