Hi,
The most expensive thing in Nicaragua is, if you see all those poor folks and can't help but help ;-)
No, seriously, you can get along with about 500 $ a month. You won't be rich, but still a lot richer than most Nicaraguans. I did live with a Family, first they charged me 70$ a month, but they cooked for me every day, and once I did calculate that alone the food was worth a huge part of my rent, and I gave them 100$ since then. But that might be a very cheap price, I know others who did live with local Families, they paid about 300$. Best if you live with Christians anyway, they will treat you the best by far. They will bless and hug and and love and pray you through anything, you won't miss home or your real family so much... ;-)
Apart from the 500$ bucks, as I said, maybe you're the kind of type who wants to go to the restaurant from time to time, so one meal in a restaurant on the cheaper scale is about 3-4$. And if you're like me, and all the cute boys who are selling their stuff on the street to help bringing their families through, and they ask you, if you buy them some food, you might well spend another 100-200$ a month to help others. But, hey folks, there is more happiness in giving than in receiving. And that's why you come to Nicaragua after all! To get REALLY REALLY happy! And, it ONLY works, if you start giving! Man, Jesus is crazy! He gave me such a crazy life! After 14 years of mental illness back in Europe!
I also know some Gringos, the guy who works in some bar, he makes about 500$ a month with his job in the bar. So he can make a living. But before you're allowed to work, you need to pay 1000$ for residence. And it's not easy to find a job, because the Nicaraguans also desperately seek one. Finding a job is easiest in domains that are runned by the Gringos, because the Gringos will give you a job easier.
A very cheap apartment (merely a room) is 40$ per month, but that's basically only a room and a "bath" and that's the price they will give to Nicaraguans, if you're a Gringo, they'll charge you a bit more. The cheapest Hotel in Granada is in the Via Cansada, and you can get a room for 8$ a night, for the time you don't have an appartment.
Food is very cheap, as long as you stay with the cheap stuff. European & US stuff can be expensive, because they have to import it. A trunk of Bananas is 0.30$, A papaya or a pineapple 0.70-1 $, rice and beans is probably the cheapest, and that's what everybody eats all the time. If they have a little money, they add some chicken to the rice and some platanas, the bananas you use for cooking.