Why are you permanently trying to confuse concepts? If you've bought raw materials and are producing goods from them, how on Earth does it happen that you have "frozen" the money?
If you are buying goods on Monday for $1000 000.- and you can only sell your products on Friday, then you have frozen one million dollars during a week. The cost of that freezing of money in your raw materials is the interest on it.
That's very elementary business accountancy, you know.
Even if so, how come that you would not "freeze" the money If you just borrowed that money to buy the same raw materials at the same time (but would have to pay interest on it at that)?!
You do freeze the money. That's why you pay interest on it !
The interest rate that is used is normally the "risk free interest rate", usually determined by state bonds. Savings accounts are usually slightly lower. So, no, you do not compare to the return on investment of drugs dealers.