When I was doing freelancing (6 years ago), the optimum price for me was 35 EUR/hour (it was attractive for new clients). Depends on project complexity, but for example hardware design of our iMX6 Rex module took 400h hours. So the price for that project is around 14000 EUR (at this moment 17-18000 USD).
Simple large boards were usually around 4-8000 USD, small CPU boards were around 12-25000 USD (e.g. CPU modules), complex boards with complete development (including software and testing) started anywhere around 60000 USD and can go over 100000 USD. Now, I need to say, this was very cheap, but as a freelancer, it was very hard for me to go with price higher than 60EUR/hour. As a freelancer, I have done also many projects for 45 EUR/hour.
Of course, I need to say I was working from Slovakia and price will depend on the country where you live.
Here you can see our timesheets:
https://www.fedevel.com/welldoneblog...dvanced-board/