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Value of a collateralized asset
You're not missing any concepts! It sounds like you are contributing a piece of collateral to the business, and you want to know a fair way to value how much this contribution of collateral is worth. Technically the economic answer would be the difference in interest between a secured loan and an unsecured loan.  So for example suppose that the business could get a loan at 17% without the collateral (maybe just on a credit card) but with the duplex as collateral it is able to get the loan at 10.5%. In principle, the value of this collateral is (17% - 10.5%) or 6.5%, because it has allowed the business to pay 6.5% less interest on its loan.