So, I have to be honest with you, almost nobody I have ever met actually understand the costs of plastic manufacturing in general. Comparative cost assessments can't be made without understanding the two costs, much of which is hidden, between conventional manufacturing and 3D printing.
In injection moulding you have several costs.
1. Designer cost - $4,000
2. Mould design - $6,000
3. Mould manufacture - $25,000
4. Factory set-up - $100,000
5. Staffing/QC/PM for a 100,000 unit MOQ run - $81,000
6. MOQ run - $160,000
That's $376,000 for your first 100,000 items. Add to that transportation, which is around $2,500 and shop overheads and your break even cost is in the region of $3.80 per item. But you must find $380,000 or so just to start!
Your second 100,000 costs $160,000
Your 10th costs $160,000
So eventually, the price of this comes down so that by the millionth item you procured, it's $1.82 per item.
What you will see organisations do, is price the design, injection mould, overheads and everything else into the LONG TERM price of the item. It's an amortisation.
So instead of charging the customer something like $5 for the first 100,000 they charge $2.50 making a loss with the view that the long-term success will bring in more margin for the same price. That's what actually happens in injection systems/conventional manufacturing.
With 3D printing, there is no initial setup cost bar a minimal slicing and prep cost. Meaning there is no way for the designer to amortize their $4,000 into the design if they are only selling it at the cost of the injection molded part. Because the per unit cost to manufacture through 3D printing is more expensive (1kg of filament is 70% more expensive than pellets just to start) and there is no huge upfront cost, which is obviously a bonus for everyone, but the drawback is there's nothing to then return it later on, by reducing that cost because that cost is not there to start with.
So broadly 3D print designers get ripped off all the time! Especially for single item commissions or small runs.
You have to fundamentally change the economics of it.
Now, the reality is the only way people can show support to a service is by buying. Most people don't have the following to share in return for an item or design and in all cases almost never returns any sales to the designer in a way that justifies them doing the work. But there are enough people out there at the moment trying to make it work that customers will get comfortable asking people to do work which is effectively done for free or very close to zero, which is unethical but mostly because they have no idea how the pricing of design is amortized over the entirety of a manufacturing or production run.
This leaves you with a pair of simultaneous equations. One for the injection molding system and one for the 3D print system. The crossover point is when you should be looking to get injection molding done if you using 3D print for small run, trials, or new innovations.