Hi There federations does not change much about how we bill. With federations, you can create large scale databases that span multiple federation members.
Each federation member is a sql azure database and can be controlled with the same SKU model you have today (web or business). You can choose to heavily distribute to small 1GB web editions or choose to consolidate into large 50GB business editions. Obviously
you are not locked into any distribution of data and you can redistribute your data and repartition with SPLIT any time and change the SKU of the members. In SQL Azure, you get billed on the collection of all databases you use in the system prorated to a day.
In the case of federations, the same logic apply; the cost of the federation is the collection of its federation members.
Hope this helps.
-Cihan Biyikoglu - program manager - sql azure