There are two things to consider. The actual OB and then the hospital.
Hospital in cash will run in 6-9K range (This is for an uncomplicated vaginal birth, no epidural, estimated). An uncomplicated C-section birth can run up to 12-14K.
The OB will vary greatly depending on how many visits you have. Call your OB, some of them do a standard flat rate for pregnancy for families without insurance. Others will charge per visit. Make sure you talk to a billing person. I know that the price my doc's office quotes is cut in half for cash patients. For example, a typical office visit is $300. If you pay cash, you'll pay $150, If you have insurance, they bill the full $300, insurance MAY pay $175 and then you have your office co pay of say, $15.
Bear in mind that any complication at all increases the prices of all of the above exponentially. ultrasounds, High risk OB, more frequent visits or tests, increased monitoring in the hospital, need for a c-section.
An epidural alone can cost $1200-2000 depending on the anesthesiologist, med and equipment they use.
Here is a link with info on free-low cost prenatal care. It has the number to call to get the local health department which to help you get in contact with who you need.
http://www.4woman.gov/Pregnancy/three.cfm
Have fun TTC.
******** Just found this link. It's what insurance was billed for births nationally in 2004. Check out page 9. This is an average of national charges. With all of the breakdown for each of the main components. Has gone up some I'm sure, but not too much.
http://www.kff.org/womenshealth/upload/whp061207othc.pdf