I've had to really think about this!
When I had Angus nearly 2 years ago, I had to pay $130 for first visit to OB which included 1st u/s of which I got back about half. The monthly visits cost $55 and medicare rebate was about $30. A gap fee of $400 had to be paid before I was 30 weeks.
I didn't have to pay anything for the blood or glucose intolerance tests.
The ultrasound at 23 weeks cost around $160 and I got over half of that back. It was at a seperate clinic.
I have full hospital cover with and excess of $250 so when we checked out that was all we had to pay. I opted to pay that amount of excess as our monthly premiums would be lower. I know you can also have a co payment option if you wish.

Where is your OB?