Will and Catherine attended the BAFTA Brits to Watch gala towards the end of their North American tour.
Kate relied on her trusty Alexander McQueen, wearing a lavender gathered gown - I'd call it almost Grecian style - with sparkly waistband across the natural waist line. The neckline appears to be a wide boatneck style with set in style cap sleeves (it kind of looks dolman in style only very short.)
I will say right now that this is not my favorite gown for her. Kate already has a wide and long shoulder line so between the wide neckline and heavy shoulders, the gown adds too much bulky width. It almost looks like linebacker padding.
However I really like the gathered skirt since a slim body type like hers becomes so much more proportionate with the added volume those gathers provide. The volume of the skirt helps even out the wideness of the top, I am just not a fan of the heavy neck and shoulder line on her.
Since I am almost sure this gown was custom the only price approximation I can muster for an Alexander McQueen evening gown is obviously well into the thousands.
J.S. Collection has come out with a similar light lavender pleated gown in polyester chiffon, with one retailer even outright calling it a "Royalty Inspired" look. It is very true to the original inspiration with the exception of looking just a bit more sheer in the bodice.
This gown retails for ≈ $250 U.S.