I don't normally stick my nose into "pro" threads, but since you don't seem to be getting a whole lot of response, may I offer some thoughts from the (hypothetical) client's point of view?
Photography is a professional service. As with all such services, the rate, and whether or not it is fixed, and how it is broken down, will vary depending on the nature of the work and the client. In particular, it will depend on your relationship with the client, their understanding of the work you do and the product you produce, and their attitude to "fixed vs variable" charges.
The client is paying for a product, a result - not (necessarily) an hourly rate for your time. The client is entitled to expect that product to be produced, for a fee, and not for the cost to vary (greatly) depending on how much extra time you have to put in on post-processing.
A reasonable, "standard" amount of post-processing is part of the deal, but the cost should not vary if you have to spend more or less time in pp because of your skill level or mistakes that you might make.
On some jobs you will spend more time than others - swings and roundabouts. You always have to "eat" your own mistakes or misjudgements. If you budget for a shoot to take 6 hours of pp and it ends up taking 12, then that's a lesson learned for you and not a bill for the client to pay.
The main thing is, the deal has to be clear about how many images you are producing, and what will be the marginal cost of extra images - like "for $1,500 you get a portfolio of 10 fully processed images, if you decide you want more, that will be $X per image". Or whatever.
But you know all that. What you're really asking is, what works best for the client - a bill/quote of, say
- $1,200 for the shoot plus $600 to produce 10 images, or
- $1,800 to produce 10 images.
Whichever way it goes (and I can't tell you what is "most common", only the pros can do that) from the client's point of view, the key is predictability and understanding of the costs involved.
I would say, as a (hypothetical) client, that either of these charging models would be acceptable if they were properly explained. What would not be acceptable, would be "$75 an hour for post-processing (however long that takes)".
In any case, I would expect the hourly rate for the post-processing to be lower than the rate for the shoot itself. Daily rates for on site attendance, for any professional, include travel time, equipment costs etc. I think in general people expect that to cost more than time spent (rightly or wrongly, this is the perception) sitting in your office "writing up the results". So the client is happy to pay the equivalent of $150 an hour for you to be on site, but they're not going to come at that rate for the post work.
Sorry there's no single clear answer in there, and the pros will be able to offer better "real life" experience, I'd just like to throw in some ideas from the client's perspective.
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