I need to know how much to charge for designing a collection of leather bags for mass production
This is an impossible question to answer. How hungry are you (literally and figuratively). How much experience designing bags do you have? How much experience do you have, period?
A good starting point would be how many hours it will take you to do the design in hours multiplied by $85. If you’re a rockstar designer, double that or more. If you suck, well…they’ll figure it out quickly and price won’t matter.