I believe you'd basically get the normal rebate of around $80 back; regardless of the type of ultrasound (whether it be 2D, 3D or 4D). That said, the limitation on rebate from Medicare is in the number of ultrasounds you're allowed. You can only have one from 18weeks plus (IIRC). I ended up paying completely for 2 (GRRRRR!) because they stuffed up the initial 20wk one; and then the doctor ordered a 32 week one because she was worried Maddy wasn't growing sufficiently. I got a rebate on my first 18 week+ scan.

If you're concerned, either check out the Medicare site, or give them a call. They'll be able to tell you what you can expect.