I did a test on this. Before I did a test, I had 229 posts and 14 minerals. I then go and edit one of my posts. I don't mess with my text at all. Instead, I clcik on "edit", and then "submit modified post". I then go check on how many posts and minerals I have. I have 229 posts - but 10 minerals. During the test, I didn't use any minerals on the store, SEN's army, the arcade, or zerg pets. (How can I use zerg pets, when it is disabled?
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This test has led me to the conculsion it costs someone 4 minerals to edit a post. Is this a bug? (Yes, I do use minerals.)
EDIT: Before the test, I put myself on vacation, meaning I couldn't be attacked. I wasn't using anything before the test that would take away from my mineral count. Before I edited this post, my mineral count was at 18. Now, it most likely will be 14.