Fixed costs: Costs of concrete mould, business costs, leather costs
Per-unit costs: Costs of producing each additional unit, in this case the additional concrete used
Your per-unit costs should not be a figure of how much each unit costs to make, instead you should try to split it up into fixed and additional per-unit costs so that your costs don't overlap and the cost savings can be passed on to your customers.
Same goes for your earnings:
- Fixed earnings: Your design fee
- Per-unit earnings: This can compensate you for any production or packaging work you have to do on each product, or it can simply be a margin that you wish to keep for yourself.
Again, the margin you choose to keep for yourself should not be calculated twice, as you have already calculated your design fee previously.
Hope this helps! Here's more info on how the price graph works: For fixed/overhead vs unit (both costs and profits have them), the general pattern is that the former makes your curve curvier (there's a greater price reduction effect the more people buy), while the latter uniformly raises your curve up or down. At 0 fixed costs and 0 fixed profits, you essentially get the price graph of a typical shop (which isn't what Melvin and I are interested in).
You will get this amount as long as your project hits the minimum. Treat this as compensation for your own overheads, such as time spent designing, prototyping, liaising etc.