As you are dropping your child at playgroup, the child isn't in the childminders numbers until collection time (though reasonable for childminder to charge for time it takes to get to playgroup).
Some childminders operate sessions, rather than per-hour, so if you want care at any time during a session then you would pay the full session fee.
Your CM may find it hard to fill the space created by your child not being there whilst at playgroup. Therefore it may be charged as a childcare space per day, rather than hourly.
What happens during holiday periods, would you need childcare then? If you were only to contract childminder for say 12-6, then extending that time during holiday periods may or may not be available.
Every childminder will operate differently, so there isn't really any norm. Falls down to what is agreed between childminder and parent, such an agreement would be written in a contract.
Instead of looking at things on a per-hour basis, how about looking at it on a yearly basis. Work out how much childcare will cost you over the year, how many hours of childcare that is, then decide if that is reasonable. Compare the figure with other childcare providers.