Best information I've had is that the 16GB cards are likely to be around £400 - http://www.creativevideo.co.uk/public/view_item_cat.php?catalogue_number=sony_pmw-ex1 . Which translates to about $811, though a lot depends whether that's a tax inclusive or exclusive price.The 16GB SxS is 700 Euros, not 700 dollars. 700 Euros = 950 US dollars. Or, based on the limited information we have now, SxS cards appear to be more expensive than P2 cards.
It's also important not to confuse European pricing with US pricing. We traditionally suffer a big markup in actual terms compared to the US, so even if it did sell for about $800 (£400) in the UK, it's likely to be substantially less in the US.
As an example look at prices on the same site for P2 cards - £489 ex tax for a 16GB P2, or virtually $1,000. Even assuming the worst possible case (SxS cards are £400 plus tax), that would still mean that P2 has a 20-25% premium over SxS. And for a given size card, the EX will record 3x as many minutes.