So, Imagine you are in the future (let say 2016) and you don't manage anymore your AGS and only consume it as a Service (Read ArcGIS Online), imagine your wish to publish some map services and you want to know how much it will cost you. Here is the answer: ArcGIS Online | Credits Estimator
Visit this site to have an idea of the costs involved in run a webapp with AGOL : ESRI Conservation Program: conservation geography, activism and multicultural social change
So is AGOL the future of web mapping? When AGS start to be economic viable? Is AGS an alternative to AGOL just for security reasons?