According to a help article by Google:
There is no charge for requesting a quota increase. Your costs increase only if you use more resources.
So by going from that, you should not be charged if you increase your quota but instead if the number of requests to the server goes past the original quota. You will be charged by the number of requests to the server so if you hit 301 read requests per minute then you will be charged accordingly, but not the same as hitting 600 requests per minute. If you stay below the 300 read requests per minute quota then it appears that you will not be charged.
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