I'm going to take an alternate view.
Stock photography, almost by definition, is generic. It can take a lot of skill and work, and certainly some creativity and visual sense — but ultimately, it's something that can be reproduced by someone else with the requisite skill. It's not likely to be a one-instant-in-the-universe work.
So, sure, if the client wants all rights to the image, go ahead and consider transferring the copyright. Maybe license back the right to use it in your portfolio, or maybe don't. Factor that in, and everything else, and come up with a price that's fair and reasonable for yourself and for the client. You could offer both the copyright transfer option as well as an option to retain copyright but grant a wide license, at different costs.
Here's an example form I found in a random web search, but I would certainly instead consult with a real lawyer, because there's a whole lot of legalese in there, and legalese is code, not English, even when it looks to be.
Of course, if you feel that the work is particularly unique, you may not want to go this route. But if it's people in business suits doing business things, don't worry about being so attached. It's only copyright, not your immortal soul.