It could be nice if its terabytes as you said but i doubt it will be like that.
Since Tableau Online is hosted in Amazons AWS, it could cost a lot though.
However, since Engineers at Tableau knew that they will provide such
service to thousands or even hundred of thousands users,
I could tell they knew their server requirements will do have lot of workload, traffic and bandwidth and they
already estimated,predicted and calculated how much resources they need to accommodate
Note that Tableau used not just one Server but a cluster of servers and their Tableau System applications.
So I think you should not need to worry about what you're going to do with their servers, its just
a little task for their well made machines.