Disclaimer: I do custom pedals rather than PCB development, but I think the principles below are sound and transferable to your situation. I've also used this process to price equipment repair, music lessons and performance.
When I think about pricing, I prefer to base it on my costs rather than the market, so here are the things that go into my equation for a job:
1. Time - set a value on your time per hour, and multiply by the hours needed to develop the project from scratch. Got it partly done in advance? Great, that will offset the other occasions when you will spend endless hours figuring out how to do something and would be embarrassed to charge the customer for all your time. On the average, it works out about right. Technically this is labor.
2. Resources - add up the cost of your tools and equipment and assign a fraction of the total to each job. Technically this is depreciation, and allows you to replace/upgrade when necessary. If you say 1/2 of 1% of the total value, you'll have 100% cost recovery after 200 jobs.
3. Materials - your cost for vendor products and services for this job. Includes board manufacture, customer-related shipping, PayPal fees, third party fees if you work thru eBay/Reverb/Etsy, etc.
4. Profit - whatever markup you want here, from zero to ridiculous. I don't do this for a living, so I usually add zero. 100% is common for commercial products.
Once you have that total, you can either quote the entire job, or break it down by board. But I would suggest a flat amount for the first order whether it's one board or 20, to cover all your overhead and development. Then you can charge a modest incremental amount for reorders of the same board (without changes) to encourage repeat business. Changes and revisions would invoke additional time and possibly other costs. Hope this helps you think it thru! Mike
PS: I love the graphics on your pedals! Killer.