Yeah that makes sense, I mean it will probably be a bit more than that per month when it's up but half when it's down (just fyi, when AWS instances are down you still pay for the EBS storage).
If you are trying o save a buck here, double check on that otherwise you will end at the same cost level but after spending resource on migrating.
If you want a good hack you can always download the data and then just store it locally (i.e. half the migration job) and just reduce the number of users when the account is not being used, probably less work and cheaper 🙂