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There are several competing models for estimating the cost of equity, however, the capital asset pricing model (CAPM) is predominantly used on the street. The CAPM links the expected return of a security to its sensitivity the overall market basket (often proxied using the S&P500).
In general I would take the minimum rate of return, which is a relativly risk free security, which might be bonds issued from stable souvereigns. To this minimum rate of return I would add the premium you might expect for taking the risk of your investment (difference between risk-free rate and market rate) adjusted by the beta coefficient reflecting the specifics of your industry/investment (Beta). In case you are not sure if your investment would deviate from other comparable investments you can set Beta =1. However, this strongly depends on your investment. You can check on public available database to see some comparables for the Beta. To reflect the different weight of different aspects of equities, I would take weighted average cost of equity. This depends on the capital structure of your company. From an islamic point of view, one might argue it can be an issue to take an interest driven risk free security as a bases for your calculation as you would not be allowed to invest in such securities. (Typically bonds are used). However, if you would follow such argumentation, you would find Sukuks with sufficient history to reflect a proper rate of return and show a relativly strong stability.
The formula is:
Cost of equity (re) = Risk free rate (rf) + β x Market risk premium (rm-rf )