Here's the reply from Molly Zurn, working in Esri's Software Product Release Dept :
It does not cost credits to publish the web scene. It does cost credits to store the scene on ArcGIS Online and the cost depends on the size of the file. The rate is 1.2 credits per 1 GB stored per month. See our credit page for more info: http://www.esri.com/software/arcgis/arcgisonline/credits. The page also includes access to an interactive credit estimator.
