Augur's decentralized oracle bases its self worth directly on the USD market cap, irrespective of the price of an ether. The "5x multipler" has been switched to a 7.5 times multiplier in the new version of the white paper.
The formula for estimating REP value is to determine the total open interest of REP (the total amount of ETH or Dai bet within the Augur system at any given time). Then, multiply that value by 7.5. Then, divide by the total number of REP tokens (11 million).
Let's assume REP is able to capture 1% of the current open interest of sports betting ($40 billion) within the next 5 years. This assumes a yearly open interest of about $400M. Because this is a yearly value, the actual value held within REP's 7-day windows at any given time is smaller. We must divide this yearly value by the number of weeks in a year, giving us about an $8 million dollar open interest value.
$8M * 7.5 = $60 million.
$60 million / 11 mil rep = $5.45 REP
Adapted from: https://www.reddit.com/r/Augur/comments/6w9ey6/how_to_value_rep_in_the_new_model/
To play devil's advocate for a moment, Augur can be used for a lot more than sports betting... It is left as an exercise to the reader to perform the same calculations beginning with the $1.2 quadrillion dollar derivatives market ;)