Normally there would be two fees, but there is something about DAI that doesn't require a fee. Unless either the site is somehow burying the fee, which I think is unlikely, or I am overlooking something. You get a little confirmation window whenever you prepare a conversion of units from Coin A to units of Coin B, and there appears a conversion fee that Coinbase imposes onto the user. But when you convert to or from DAI, that conversion fee is nothing.
So I would just default the value that I have, around $65, to DAI, and then convert it to another currency that is experiencing a dip, wait for that currency to rise, however slightly, then convert it back to DAI, and I don't seem to be suffering any loses from doing so. Its all very low stakes, so its tough to really pin down if any increase in value is actually due to successful arbitrage, or if it is due to something else.
edit: there might be other currencies that don't require a conversion fee. Not sure if it is a stipulation that is inherent to the currency, or if it is a condition agreed upon by the currency and Coinbase.