1. A method for billing customers in a multi-tenant billing system, the method comprising: computationally measuring usage by a customer of resources in a cloud-based computing system; receiving, from a reseller of the cloud-based computing system, (i) a set of values defining a type of resources offered to the customer and (ii) a billing method comprising executable computer code specifying a procedure for calculating a bill for the usage by the customer; providing a secure execution environment dedicated to the billing method and within which the billing method is executable without communication with any other billing methods executing in any other secure execution environments; computationally verifying, in the dedicated secure execution environment, digital signatures associated with the billing method to thereby authenticate the billing method; and executing, using a computer processor, the received executable computer code of the billing method in the dedicated secure execution environment, using the set of values and usage by the customer as input, the received executable computer code comprising instructions to (i) retrieve the measured usage via an application-programming interface to the dedicated secure execution environment and (ii) determine, in the dedicated secure execution environment, a charge to or by the reseller for the usage by the customer.