3months,
Where in the US will you be living? I ask because the insurance and midwife/homebirth situation differs from state to state.
If you want to get a rough idea as to how much self pay insurance would cost, I would look at Blue Cross/Blue Shield and United Healthcare to start. These are 2 of the big insurance companies. They should be able to give you an estimate via e-mail. Another option many people don't consider it AAA--the auto club people. Believe it or not, they often have very competitive rates for people who are self insured.
Also, many hospitals now offer their own insurance plans. You would have to contact hospitals directly in the town where you plan to live.
There are also growing number of physician practices that are cash only. Google for the name of your city plus "cash only" and see what you come up with. Generally these doc's charge significantly less than others. Sometimes it's very cost effective to get a catastrophic insurance plan for hospitalization only, then use cash only doc's for your primary care.
If you don't get maternity coverage, I would do as expat mom suggests and consider a homebirth or birthing center, or negotiate with the ob and hospital up-front. My sister negotiated with her ob and hospital when she was pregnant, and her total cost was about half of what our insurance covered delivery was.
Good luck!