Handle cost changes in billing statements

At the moment, we are assuming the cost of all units are the same.

A similar, related issue that will probably get solved by the same fix: https://github.com/praekelt/vumi-go/issues/1063
Super bikeshed / minor semantic niggle -- I think ``else count`` should be ``else 0`` since although they're numerical idental, we're really returning a zero cost, not a zero count?