Outside of basic accounting finance you should really consider risk unless the you're dealing with a kind of security like treasury bills. 

However, as long as there is enough historical data to establish a loan default rate, it should be relatively simple to calculate risk and the required amount of interest required by people taking out the loans in order for the loan giver to still make a profit. 

There is a whole concept in finance where risks are put into two categories of risks specific to an investment and market risks. Risks specific to an individual investment are basically negligible when you make enough loans. Which is the idea behind the collateralized debt obligations (CDOs).