has anyone done this or does anyone know this can be done to get around the inflated purchase prices of crypto through coinbase?
Yeah, GDAX traders don't pay Coinbase's "conversion fees." When you buy and sell on Coinbase, you get GDAX's ticker price plus the conversion fee (1.5-4%). When you trade on GDAX, you pay 0.25% taker fee and 0% maker fee. It's slightly higher for LTC and ETH. So obviously, the difference is massive.
Also, GDAX does not have any network transfer fees. People are complaining about Coinbase withdrawal fees because they don't realize they can withdraw from GDAX for free. My thinking is that Coinbase really needs liquidity for their order books (look at the BTC and ETH flash crashes earlier this year). So they cater to their traders, especially on maker orders, while they screw the noobs with Coinbase fees.