I lived in Bali for one year not so long ago. 2014-2015 to be precise. I had some fn times and some not so fun times but overall it's well worth the experience if you can manage it.
So first things first let me tell you how much i roughly spent in a year there.
$10,000.
Here are some helpful tips for the long term traveler in Bali (note these prices will be significantly less in other parts of Indonesia)
Scooter Rental
500,000 Rup per month
Monthly food budget if you are a big eater, eat at local warungs for dinner,
don't go to western sytle restaurants and make your own breakfasts
3,500,000 Rup per month
Accomodation in a small one bedroom apartment somewhere near kuta
3,000,000 Rup per month
Travel insurance (always get the no excess fee option)
750,000 Rup per month
Petrol costs if using a scooter as main transport
150,000 Rup per month
Sim card phone credit and internet monthly cost
(assuming low internet usage of about 5GB per month)
120,000 Rup per month
Miscellaneous other costs
200,000 Rup per month
Surfing cost such as parking fees/boat fees board repairs
150,000 Rup per month
Visa fees, departure tax and associated costs of stay
(assuming you stay on either tourist visa and extend or the social visa
and do everthying yourself at immigasi)
400,000 Rup per month
TOTAL
8,770,000 Rup per month
Now this is about $10,000 AUD in current exchange rates but given that this information is relevant to 2015 inflation is going to play a part, i would estimate about 5% for each year to be safe. These are rough figures but based on what i spent living there. I kept track of every Rup i spent on everything so i can estimate fairly well on a lot of expenses. Also note i didn't spend money on alcohol, smoking or hookers!
One good way to offset rental expense is to find a place with more than one bedroom and take a risk renting it out and trying to find someone to share with you, if you use places like Airbnb its fairly easy to manage but you have to keep it low profile as its probably not legal to earn income through rent on a tourist visa, still it's not technically income i guess if you are just offsetting your rental cost.... assume anything you do where you pay less than the norm is probably illegal in Indonesia and keep a low profile anyway.
I knew a guy who payed 1,000,000 a month rent living way out on the Bukit in a very cheap room with shared amenities but i would say this is an extreme way to save money and could be more a hassle than it's worth, still those options do exist.
I was living in Seminyak which is good to get to Kuta reef (20 mins) Serangan and Cangu (about 30 mins each) however the Bukit took about 1 hr or so on scooter to get to. I was 5 mins walk from the beach and in a back street with little noise from roads but the tradeoff was some really really bad congestion during peak season which is common around the inner seminyak small streets area (and most of Bali to be honest) and is one of the main reasons anyone would not choose this area.
Anyway hope this info helps a bit.