Moving to Costa Rica from France

Congratulations!
$600/month would be a livable salary most places in Costa Rica.  Monteverde is a tourist place, and so many of the basics (on the surface) are expensive.  You will have to dig to find the deals.  Some examples:  a casado (plate of food, special of the day) is $5-7, taxi to cloud forest is $8.  Entrance fee is $5 for nationals, $17 for tourists.  A bus to San José is about $5 (one way).
You would need to shop as much as possible at the feria (farmer's market, for fruits and vegetables, some other items), cook most meals for yourself.  Use the shuttles/buses or walk to get around.  See about renting a room (maybe with board and laundry service) from a local.  Skip tours; find out from your fellow workers what they do to economize.  Make sure your job includes caja (medical care) and aginaldo (Christmas bonus), which it should if it is on the up and up.
Monteverde is high, and so is colder than much of the rest of Costa Rica - take appropriate clothing.
good luck!
Julie

Thank you Julie! So you think I could "survive" with USD600. It's true that it's hard to realize being based in Europe right now! Can you tell me more about the weather in Monteverde? Is is always "colder" up there? How should I be prepared? Rain clothes, rain boots? But does it get warmer at some point?
It would be a big move! I'm 29 and have a stable job here - but was looking for some change! I speak French, English and Spanish and I have visited CR twice (Puerto Viejo / Cocles mainly).
Do you think I'd be able to save any money though? I'm thinking plane tickets back home (Paris, France) at least once a year...!!!
Thank you SO MUCH for your help I really appreciate.

hey Nousse, you're welcome!
I think you could survive, but not save much of anything.  If an airline ticket to Europe from Costa Rica is $1200, you can see that it would take an awful lot of effort to save for that.
If you rent a room w/ partial board for $10/day (I'm guessing that's possible, but you would have to check w/ your future employers; hopefully they can help you with this sort of info), that right there is half your income. Add perhaps another $5/day for food, misc, and the occasional beer, and that leaves you with $150/month.  You will probably have to go into SJ once in a while, and you might like to take some tours and/or look around the country some more.  If not, you might be able to save enough in a year for the ticket, but not much else.
re "colder" - often the highs get to the 60's (F), and inside at night, right at 60 degrees.  You will definitely want a light raincoat and rubber boots (although you could get the boots here easily enough).  Bring sweaters/sweatshirts, and warm PJs. The temperature is essentially the same year-round, since it's in the tropics - elevation is what determines temperature.  No wide swings.
good luck!
Julie