To get the money back for a NT scan from medicare can be a complex issue...the referral has to be worded right and there MUST BE A REASON for doing the scan. If you are not in a high risk group (age, fam history etc) there is a chance that you wont get anything back. I used to work in a bulkbilling ultrasound clinic and there were heaps of hoops that we had to jump through to get medicare to pay up.
But as the PP have said...its a personal choice. It also just an assessment of risk ie 1 in 12987 chance of having a child with trispmy 13,18 or 15.
Good luck with whatever decision you make.