I've sold photos on and off for 20 years (on a direct basis, companies contact me via my portfolio), and I've generally gone for a rough "$5 per employee that the company has" rate. A 10 person startup wants one of my photos? I'm happy to give it to them for $50. 200 person company with a marketing employee calling me? $1000 sounds about right. Microsoft wants exclusive use of a photo? $700k will do just nicely thanks ;)

You can't always accurately assess this of course, so it's mostly done by gut feel after learning a bit about the company, but it's done me well so far and I've never had a company turn down my price, nor have I ever felt like I've been taken advantage of.