One of the main values of Sex.com is traffic. That is something Sex.co does not have.
For the most recent month Compete.com shows 211K visits for Sex.com and 220 for Sex.co, not all that impressive.
Also, the Alexa ranking is over 1M+ which is not very good.
Outside the .COM, here are the other two highest "Sex" sales -
Sex net $450,000 May 06 Moniker/Traffic
Sex info $202,000 Aug 07 Sedo Auction
I don't think it will meet either of those prices.
They are clearly just trying to ride the coattails of the .COM sale, but that sale really has no relevance on the value of Sex.co
Brad