We can do that. I think it might make the query we use to calculate the message cost simpler too. The advantage of having them separate right now is that it is more intuitive to add cost overrides for an account via the Django admin inline forms, but if that UI will eventually move to the main Vumi Go interface its irrelevant. I have used South migrations for the tables. I don't think Django supports composite keys, perhaps there is a way to add them to the migrations.