An hourly rate means nothing in this situation, it doesnt matter if he can take all 15 shots in less than 15 minutes.
The basis here is the usage. He will have 15 shots used for advertisement in a direct mail brochure reaching 50,000 people. Its the USAGE that will drive the rate in such instances, not how long it takes you to do the job.
The greater the worth of the photos to the client, the greater the worth of the photos to you. Hence advertising is worth more than editorial because its going to make the client more money. Hence a national advertising job is worth more than a local one, as it will reach a wider audience for the client. Hence a brochure with 15 shots going to 50k people is worth a heck of a lot more than one with one shot going to 10k people.
Its USAGE and that is what photographers need to wrap their minds around, not how long it will take them to take the photo.
And BTW, there is enough info here to know that $500 is waaaay too low. 15 photos, 50,000 circulation, and advertising is about all you need to know to understand that its too low. Any more info helps you fine tune a quote, but $500, or even $1000 is too low.
Additionally, when I think someone is undercharging significantly that will skew the value of photos in the industry, I think that is a perfect time to pull out the soapbox. I think I am saving it for the right time.
Rather than folks think people like myself or chtgrubbs are up on a soapbox, it would be nice to see them actually listen and take to heart valid information that might help them actually learn something and allow them to make some money in this profession.