I guess you'll have to decide for yourself if it's worth it, but you'll see in the Illustrator Training Manual that silhouettes are almost always priced at one credit. If you haven't already read the pricing guidelines, you might want to familiarize yourself with them in order to avoid future frustration.

Here's a link:
http://www.istockphoto.com/illustrator_6.0_pricing.php

I realize there are 6 people, but the pricing is somewhat objective. If you look around at some others (nicoblue's is a portfolio that contains many of this type of illo) and still feel that your illo was priced out of line with others that are similar, then you can send an inquiry to have it reconsidered. Here's the info on how to do that:
http://www.istockphoto.com/forum_messages.php?threadid=49716&page=1

Good luck!

MIchael, if your file is useful and what a designer is looking for, they will not let price stand in the way. Your file is going to do very well I'm sure. There is a similar one, at 5 credits, that has had consistent downloads. I definitely think your file is worthy of 5 credits.

When I set out to do an illustration, my goal is usually to create a 5 credit file. Unless the 1 credit file is especially unique (I've gotten that lucky a few times) I don't feel the low volume sales of 1 credit files generate enough income to make it worth my while. Some people have the ability to churn out tons of fast-selling one crediters, but not me. I shoot for the 5 credit mark as they sell more frequently in my portfolio than my 10-credits and they make me more money than my 1-credits.

There is a great article on this topic, how to spend 5 more minutes *after you think you're done* to really put the finishing touches on your file that push it over the line from "just enough to get by inspection" to something special. I'll see if I can find that link and come back to post it.

Edited to add:
Here's that link. Check it out!
http://www.istockphoto.com/article_view.php?ID=180

well, maybe now with the new pricing that can be a 3 credit illustration

I don't understand why some illustrations are being sold for 1 credit on Istock
for example this image: http://www.istockphoto.com/file_closeup/people_specific_attributes/number_of_people/5_or_more_people/3452883_people_dancing_at_a_party.php?id=3452883
And its being sold for 1 credit while a photo is being sold for much more.

But the illustration isn't really one that requires skill. Is it worth the effort? probably no, but most of iStock isn't worth the effort. Hell, I make 6000x more playing poker, but I enjoy photography. If you're not doing what you love, then why are you doing it?