Unfortunately, I don't think there is going to be any hard numbers as an answer to your question. This is based on the MyMoodle dashboard
allowing a range of flexibility in their design. If I opt to go with an image rich solution for MyMoodle of course there will be a cost storage wise.
A quick test would be to check the storage prior to creating the MyMoodle for a user and after. That will give you what your overhead for storage would be and I would say to add another 10% to that figure just to insure you have a buffer.
I know this is not an answer but I hope it help.
Make it a great day.