Just like bus fare anywhere else in the world, the price really depends on where you are travelling from/to. The price of fuel has dramatically risen in Indonesia in the last few years as the government used to subsidise it so that the price didn't change, and in the end it was sending the country bankrupt. So the price of everything, including transport, has risen. But it is still very reasonable.
The best idea is to check with your hotel or a local vendor you become friendly with as to what a fair price is. Even then, there is tourist price and local price. It is just the way it is. In Bali, you will find in the busy tourist areas that bemos are not as easy to find. Mostly you will be offered chartered transport. Once out of the busy areas, there are bemos / public buses, but you need to go to the terminals. In Lombok, you will see bemos everywhere full of produce and people.