Well that depends on where you are coming from you can fly there or take a boat. A boat would be much cheaper. Bali is not that cheap though as it is a very touristy part of Indonesia. It is very beautiful though and as long as you stay away from the touristy parts as much as you can you could probably live there for about $1500 a month.
I would imagine this is dependent on where you are coming from. If you are coming from some other country, the only way to get there is by plane, although you could probably get a ferry or boat going there, which would be much cheaper. If you prefer to fly there, you will most likely land at Ngurah Rai International Airport.