Again, you'll get a variable amount per image depending on what deal the buyer has got, either by bulk purchases and/or by applying any discount codes.
I'm exclusive there, and I wouldn't recommend it to anyone nowadays UNLESS you have 'played the field' and found that your particular port does much better at iStock, or unless you will consistently have your pictures accepted as S+ thus ported to Getty (if the connector is working). Even then, there are reports of people doing much less well on Getty than they used to.
Bear in mind that your files will now be undercut by indies, and buyers can easily hide all exclusive images from view with just one click.
Credit sales are decreasing rapidly month on month, and it's pretty certain that soon iStock will be just another subs seller.
In addition, you will not be able to add any new files to the PP, so if that was an important income stream it's a consideration.  (Your old files can remain there, or, IIRC you can remove them).

ADDED: Wow! If you have made 210 credit sales since June this year with a small port, iStock exclusivity might indeed work for you, unless your previous buyers were the budget-conscious ones who exclude exclusive files from the search.
http://www.microstockgroup.com/istockphoto-com/difficult-decision-for-a-person-who-18-years-old/msg422682/#msg422682