1. A method for billing customers in a multi-tenant billing system, the method comprising: computationally measuring usage by a first customer of components in a cloud-based computing system; computationally measuring usage by a second customer of components in the cloud-based computing system; receiving, from a first reseller of the cloud-based computing system, a first set of values defining a first type of components offered to the first customer and a first billing method comprising executable computer code specifying a first procedure for calculating a bill for the usage by the first customer; receiving, from a second reseller of the cloud-based computing system, a second set of values defining a second type of components offered to the second customer and a second billing method comprising executable computer code specifying a second procedure for calculating a bill for usage by the second customer; computationally verifying first and second digital signatures associated with the first and second billing methods, respectively, to thereby authenticate the first and second billing methods; executing, using a computer processor, the executable computer code of the first billing method in a first secure sandbox environment, using the first set of values and usage by the first customer as input, to thereby determine a charge to or by the first reseller for the usage by the first customer; and executing, using the computer processor, the executable computer code of the second billing method in a second secure sandbox environment, using the first set of values and usage by the second customer as input, to thereby determine a charge to or by the second reseller for the usage by the second customer.