Hi RobD
It very much depends on what type of stock images your wife needs. Itsock ones are really cheap, but sometimes it is impossible to find a suitable image..In
http://creative.gettyimages.com the images are much nicer than on istock but the price is higher as well
I think your wife should offer to her clients both a cheaper and more expensive option for stock images. I have noticed that clients are very willing to pay more for an image if they like it a lot. You know, the graphics make a big difference..
But no matter what stock site she chose, she better check what is included in the subscription price..some clients demand exclusivity to use a certain image, and e.g in istockphoto this has to be additionally negotiated with the photographer..I used to deal with such a case and guess what - the designer who turned out to be russian requested ~ $20K for exclusivity rights for a photo which was nothing special..
So better check carefully the terms and conditions for using any stock images before buying..
Hope that helped..
Best
Chi