I agree with Powder that you should seek 2nd opinion with another Gynae.
If the 2nd Gynae's result is different from 1st Gynae, seek confirmation with 3rd Gynae.
2 Gynae : normal , 1 Gynae : DS = Keep BB
2 Gynae : DS , 1 Gynae : normal = some risk BB
3 Gynae : DS, 0 Gynae : normal = Very big risk BB
But when seeking other gynae, please been tactful that you don't reveal you have been to another gynae, if not the assessment may be skewed to the previous gynae or the gynae may take offence that you have been "Doctor-Hopping" around.
This kind of money cannot be saved as it concerns a life, your precious BB.
For me, I didn't even do a DS test on my BB because my Gynae said no need to do since my hubby and I are able to accept a DS baby.
I really can't bear to kill my child and I have seen DS children growing up happily with their parents, I'm sure I will be able to handle with my hubby's support.
But then again, my hubby is working in the healthcare sector and is of compassionate nature so the situation may be different from you.
P.S : Don't laugh at me but the moment I gave birth, the 1st thing I see is not how cute BB is but whether she has all the fingers and toes, any cleft palate or abnormalities etc.
Throughout my 9mths pregnacy, sometimes I will have this fear that my BB may be abnormal because Ultrasound is not definitely accurate and Gynae may not be able to detect eveything.
Utill BB is out, then I'm at ease.
This is every Mummy's Fear but I just have to accept and deal with them.