Rex, my personal advice would be not to go forward with this project idea. I'm a general contractor of 20 years+, and I've yet to see one done correctly and problem free, even by so-called professional basement companies.
This is an extremely complicated and dangerous undertaking, and the only way I would personally do something like this for one of my customers would be to have the home raised and moved off the foundation, build a conventional basement, and then re-set the home on top. No matter how it is accomplished, you are correct; it is a very expensive job.
Just my opinion.... best of luck whatever you choose to do.