Yes, I did it yesterday. There are very small/ negligible charges on each transaction i.e. From Coinbase (Bitcoins) to GDax and then from GDax (Bitcoins) to Poliniex.
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Yarr did you try transferring money from coinbase to Gdax and than from Gdax to poliniex ? i think through that you can save charges as Gdax is coinbase child. I heard that but not sure.