It can vary between $15-$60 an hour per image.

How that break down (let's just call it the $15 variety) is like this -each image starts the clock with a minimum charge of $15. If it takes them 2 hours, you'll be paying $30.
If they do 3 images all within the first hour, it's gonna be $45 - even though they were finished in about 10-20 minutes each - because there's a $15 minimum charge per image.

There was a company around here that did nothing but Photoshop alterations that charged something like $30 per hour per image if that helps.

If you're experienced (and I REALLY mean really good and fast), you can get on with an experienced (meaning: expensive!) photographer. You could easily charge through the roof. Their client is paying for it, it will usually be on a per image basis (freelence as opposed to an actual employee - cheaper for THEM at tax-time), and the deadlines will be tight - which is why you can justify your bill. Ever heard a high-priced photographer say something like, "I work with a really goodPhotoshop/Finishing guy..." That's what he/she is talking about.

Good, Fast, Cheap - pick 2. In the paragraph above, they go by Good and Fast - your payment is billed to a client who doesn't care too much about money - this is a small expense to them.

I HAVE heard of PS guys who charge something like $1-$2 per image for wedding shots - quickie color and density correction, maybe a touch of dodging and burning, and occasional quickie removal of band-aids and warts. This is all "under 2 minutes per image" work if you're fast - you make up the money by volume - which is usually anyhere between 50-150 images to finalize. They don't do this for the proofs as they might have well over 400 images to start with.

Hope that helps-
-Lew ;-)