The absolute cheapest way to go is to build it yourself. Get a few batteries, a few PV panels and go from there. But living in a construction site isnt always what is best for a family...
Another option is to pool resources with a group of people in your area to have a few Earthship Biotects come out and help you get started.
Are there any incentives from your state? from your county?
Look at the home you live in now... what are the biggest expenses? How can you reduce/eliminate them? Can you retrofit your existing building? Can you set yourself up now to reduce your expenses, then move those expenses to pay for your earthship?
There isn't one way to go from a conventional building/lifestyle to a sustainable one. Every person's life is unique, therefore the path to get to sustainable living for each person is unique...
Please post here so that we can help you...
thanks!