SQL Azure Fragmentation / Database Size
Matthew,

I don't have direct experience with SQL Azure but I think same rules apply here as a normal SQL Server instance.

280 mb is a very very small database and the cost of fragmentation is almost 0. Coming to the size of this small database, I don't think you can control that nor should you be worried.

The above is because when SQL Server creates a new table, it doesn't know how big will that table be. Data is stored in pages of 8 kb and an extent is a collection of 8 contiguous pages. 
There are 2 types of extents, mixed and uniform extents. Uniform extent means all the 8 pages will be used by the same object (say TableA) and mixed extent means that this extent can be used by as many as 8 different tables. Since SQL Server doesn't know how big the table will be, it will start with single page allocations until the table reaches 3 extents and there after it uses uniform extents. This is the reason, fragmentation is high in small tables and but you shouldn't be worried abt it.

Instead of looking how big is the database, you should look at how much of it is empty. I am NOT sure how this affects the billing though. HTH