Here are some sources. GDAX charges 0.3% fees for BTC/USD market orders and 0.25% fees for ETH/USD market orders [0]. They charge slightly less if you trade large volumes (millions of $) which most users will not hit. GDAX's pricing structure incentives users to place stop/limit orders which help provide liquidity.

For purchases through coinbase, in the US they charge 1.49% in fees for cash/bank account purchases [1]. They have different pricing fees for other countries.

[0] https://www.gdax.com/fees/BTC-USD

[1] https://support.coinbase.com/customer/en/portal/articles/210...