I had my daughter at 41 and I did have the amnio. My insurance at the time paid for it. I'm certainly glad it showed no problems, because I didn't want to have to answer the question you ask here "would [I] really terminate a pregnancy at 18 weeks?" Before I was pregnant I thought I would, but at that time, it would have been a devastating decision. Of course, the amnio can show other, more serious issues besides Down Syndrome. I would have wanted to know if my fetus wasn't viable, for instance. Better then than at the very end of the pregnancy.
The question of risk vs. age is an on-going debate, and understanding the question requires a knowledge and understanding of statistics (a class I'm taking right now, BTW :-) The numbers "1 out of 98" sound so scary because you can picture a room full of 100 people, and those odds sound terrible. But this is misleading, as these numbers are generalized over a large population. I told myself that those numbers meant that I had a 98% chance of having a perfectly healthy baby (turns out I did have one).
So, if you answer your own question "no" that you wouldn't terminate a pregnancy, then the next question is, is $1600 worth spending to be prepared if you are expecting a baby with special needs? Maybe it is, that would depend on your finances and your personal needs. It could also buy you reassurance if you feel you need that. (My husband needed some reassurance, in our case).
Another recent debate I read about is the small risk of miscarriage from the amnio procedure. It is a very small risk statistically, but is even that risk too large for a woman in her 40s that might have difficulty conceiving and carrying another baby because of age?
In the end it is a personal decision, after you have weighed the scientific data, and your personal needs.