As Olaf already stated, there are no additional costs for any activity on the MI.
If you restore that database and keep all those databases daily, your storage might be used to the full, which might create additional costs, but if you just have one database that gets dropped and restored every day... no worries, that won't create any additional costs.
If you are using other services to run the automation, that might create additional costs, too... depending on which service you are using, there might be just xxx runs including in the base fee, and if you use more than that.. you have to pay an additional fee for that automation... but you just asked about Restore/Drop...