Training/Billable seconds

With SageMaker at the end of the training are printed the following two lines:
* _Training seconds_: X : This is the actual compute-time your training job spent
* _Billable seconds_: Y : This is the time you will be billed for after Spot discounting is applied.

I think it will be nice to have something similar after we run a computation with dask. What I mean is:
* Total time: $T=t_{w_0}+\ldots+t_{wn}$ where $t_{w_i}$ is the total time in seconds worker $w_i$ was on.
* Cost estimative: which is just `T/60**2 * worker_hourly_cost`