I just noticed that prices for Oracle Database running on Windows Azure have now been published.
The prices for the virtual machines with the Oracle license included run from about USD 820 per month for Oracle Standard Edition (SE) running on 2 cores, to a whopping USD 9300 per month for Oracle Enterprise Edition (EE) running on 8 cores. These prices are for Oracle software only and do not include the cost of the VM, which is billed separately.
Alternatively, with "License Mobility", you obtain a license as normal from Oracle and run that software on Windows Azure by deploying a Windows Server or Linux VM and installing a copy of the licensed Oracle software. Azure is listed as an "Authorized Cloud Environment" in the Oracle Cloud Licensing Policy. As I understand it, this policy allows you to use Oracle Standard Edition One (SE1) on Azure (up to 4 cores = 1 processor license, up to 8 cores = 2 processor licenses), which would be a quite cheap entry-level option for small businesses, since you would pay once for a perpetual database license, and then just pay monthly for the Azure VM (ie just the Windows/Linux Server software, not the database).
There's also an article at ZDNet with more details.