Do you have any construction experience? If not then you're in for quite the adventure! Doing your basement yourself can be very rewarding! But it also takes a lot of time. I'm not sure what you mean by debt free; If you're going to save up to do your basement in chunks, why not just save up to do your basement all at once? The cost is determined by the complexity of the floor plan and the size of your basement, without those, it would be hard to advise you on the costs.

You can see a really good resource for building your own basement here: http://www.ifinishedmybasement.com/

Best of luck!