Backpacking in South Pacific Islands (EXPENSE)?

So, we're looking for a place in the South Pacific to spend three or four months in January of next year. The most obvious ones that come to mind are Fiji, Samoa and Tonga. I've seen pictures of all three and they are all very beautiful but we're also concerned about price. which one is the cheapest. We're not looking for luxury. We're ok with roughing it a little. How much do you think would cost to backpack throu one of these islands (JUST ONE) for about three months? not including airfare. Thank you!