Electric Showers normally run in the Range of 9kw to 12kw.
If your shower is 12kw, then 1 hour will consume 12kw, therefore a 10 minute shower will consume 12kw/6 = 2kw.
A kilowatt of electricity costs maybe 15 or 16 cents.
Therefore your shower would cost between 30 and 40 cents for a 10 minute shower.
You say you consumed 7kw during a 10 minute shower, if you were using just one shower alone, this is not possible.
For one shower to consume 7kw in 10 minutes, it would need to be a 42kw shower, and I've never seen an electric shower over 12kw.
Pulling the cord should consume no electricity, because there is no load, turning on the electric shower produced the load.
Maybe you had other high drain appliances on at the same time as you were having the shower i.e. emersion, electric cooker, geothermal heat pump, 10 electric kettles ?