The Etherum gas fees to complete an Augur order currently average $4.32 per trader at the current standard gas price suggested by ethgasstation.info. The average gas used was 666022. That is >31x the gas used by a standard transaction to transfer Eth.

This transaction cost can be reduced to pennies in the future when scaling solutions like Plasma become available, but until then, these transaction fees hurt the value proposition of Augur vs. existing markets. Especially when you consider the risks that Augur's security is yet to be proven.

The fees make low value trades, less than a few hundred dollars, economically unviable. The bigger the trade, the less the effect of the $4 in fees, but early on, most user won't want to participate in high value trades until Augur has proven secure.

IMHO this will lead to slow adoption early on (which may be a good thing in the long run). The decentralized sports betting markets many users first envision on Augur are unlikely to catch on as fees will not beat those available on established sports books. The most likely markets to have any volume will be Alt coin futures markets as they don't face competition from anywhere else and traders are more likely to be interested in higher value trades which mitigates the fee problem.

If Augur does gain momentum, or any other dApp creates high transaction volume the problem will worsen as gas prices increase.

Below are links to the transactions performed on the Rinkby test net where the 11gwei gas price was used. This was calculated against the current $590/Eth price to arrive at the avarage $4.32 cost.

Taker Buy Order: https://rinkeby.etherscan.io/tx/0xd16c2351e9bc8bd0a8d434ba2e90a5ffa520ef580b607f24b7549a8d73f2390d

Taker Sell Order: https://rinkeby.etherscan.io/tx/0x0f8b6e09e6a10884aa354988d821358c4f779c31f56265027d4cd03a79c170a2

Maker Taker Buy Order: https://rinkeby.etherscan.io/tx/0xe061456921dfb6d543de3429142fa41057dbc6ccf3a5b9e64e1b5ce9ce25f701

MakerTaker Sell Order: https://rinkeby.etherscan.io/tx/0x2f20e227aa2669a4050ba474a0023da21756821c774012ce87cefb9293ddbdd6

Edit: The $4 fee applies to orders, not just executed trades. So a user has to pay the $4 to place an order that may never even be filled. THIS COULD DEMOLISH LIQUIDITY.