Ask: I've been approached by a fortune 500 company. How much should I charge?

Quick consultation here, I need your help guys. I've developed a Facebook application for pages. It's been seeing some success. I've got a white label version that charges 19.99 yearly.

I've been approached by a big company (fortune 500 ) and am now setting a skype call with their social media department.

I am not sure what I should offer them, and what budget I should be aiming for. Should I offer them a white \ gray label solution? How much should I be charging.

Thanks!