How do you work out what DocumentDB will cost?
Well, 1 RU corresponds to a GET of a 1KB document. You then need to know the specifics of your workload such as number of documents, avg document size, how the documents are accessed (query patterns), and how often documents are updated.
I suggest loading your data and running your standard queries in the Azure Portal (see below) to see what RU’s your workload generates, and use that in your calculations.
Rolf also has some comparisons to SQL Server queries and some hints with integrating SQL Server with DocumentDB.