Well...if you took each manufacturing project as a seperate venture then you could simply treat minerals invested as shares in that project.
You assign a value to each mineral in shares based on average market value.
Tritanium 1.5
Pyerite 4
Mexallon 12
Isogen 115
And so on. (I would post the others, but I don't deal in them yet so don't know the average market value of them, but whatever, you assign a value to each mineral...mkay?

)
Then you figure out the cost to manufacture (Renting the facility, any applicable taxes and other accociated costs)
So lets say you plan to make a run of 15
Whatsits.
The total resources needed for all 15
Whatsits is...
150,000 Tritanium (225,000 shares)
20,000 Pyerite (80,000 shares)
2,000 Mexallon (24,000 shares)
150 Isogen (17,250 shares)
5,000 ISK in manufacturing costs (5,000 shares)
This gives you 351,250 shares for the project.
Investors get shares based on what they provide.
If you sell the
Whatsits for a total of 1,500,000 after taxes and broker fees then each shares pays the investers 4.3 ISK (1,500,000 devided by 351,250 shares) (I suggest rounding it down to an even ISK and allowing the crafter to keep the fractioned ISK as profit)
-LA