Hi Kevin,
I'll try to answer your two questions here.
1 - Cloud rendering and credits:
When you start a cloud render you will see section at the bottom of the cloud rendering window call "Cloud Credits"
Generally speaking, the cost of renders is based on either size or quality or a combination of the two. The more computer horsepower needed to create the render the more it costs.
For example, a Standard quality screen size cloud render costs 0 credits, a Final quality render of the same size costs 1.
You will see the numbers update as you make changes to the custom settings or the preset settings.
2- Rendering alpha channel for the background:
The capture image dialog is a bit confusing. You can create an alpha for a screen grab of the regular sceen, but this feature doesn't work when you grab a local render.
You can render in the cloud with an alpha channel.
Hope that helps.
Colin
Colin Smith
Sr. Product Manager
SketchBook
Alias Create VR (aka Project Sugarhill)
Automotive & Conceptual Design Group