I have looked the source and it seems it is not possible to print the price per 100 pcs.
Fields like F_SECISIN ("Securities ISIN Code"),F_ROWPRICEREBATE ("Price including Discount, row") ignore field argument=5, which should round the field value to 5 decimals.
So, there is no solution without programming?

Example:

Price for 100 pcs is 5,14 Eur.
We can use price factors or quantity factors to get the correct price in the invoice:
If we sell 15000 pcs, then 5,14*150=771, but on the printed invoice we get Unit price 0,051 (max 3 decimals)
0,051*15000=765
We get 771-765=6 euros difference. Customers seeing this invoice will be confused.