... now wants to produce one of them. ...
It was 24 hours for one concept.
Along the lines of where iab was going.... at 24 hours per "concept" I think you would better serve your client with hand drawn work ... and then bill the "24 hours" (or whatever) on the development
of the chosen ideas. Basically, $1,200.oo is a helluva lot to pay for one "idea" (even if it does come with surfacing data); for $1200 I'd prefer to see a dozen, or more, especially since ...
"Functionally is not anything special, just a cool ornamental design, not protected by any design patent or anything."
But $50, 60, 70 ... $150 per hour doesn't really make any difference.
make a difference is what it costs you to do business. i.e.
Are you renting a studio, or working out of your bedroom? Are you purchasing E&O insurance to protect your business? Are you purchasing life insurance? Do you have health insurance? Do you have a business license and are therefore required to pay unsecured property taxes? Paying utilities? Making a profit (on top of your wages)?
In other words, you need to do an analysis of your costs.
Otherwise it's a hobby...