"Seated" in C? Are you on a paid business class ticket or have you upgraded to NC class from a coach fare? If you're on a paid ticket, C, D, or Z fare then yes you can upgrade for cash at the gate and it'll likely cost under $500 (but definitely less than $800). If you're in NC then you can ask but most likely you won't be able to upgrade.

Edited: Recent reports here: http://www.flyertalk.com/forum/unite...-here-137.html

show the price as just under $400 from paid biz to first on PS. Of course, there has to be NF space available that day for you to be able to upgrade.