Like anything, it depends. I'd rather not say too much about SG's pricing publicly (since I don't work there and don't speak for them) but it's similar to other agencies in the same market. Ballpark, that means 10k-30k depending on your company's reputation, the scope of work, which people from the agency are on your team and compensation structure.

You don't really pay a whole lot more for higher quality agencies (I could go into theories on why but will save that for another post). Companies almost never hire bad agencies because they can't afford good ones. Far more often it's because they don't know how to find (or identify) the good ones or because the good ones don't want to work with them (it's a big risk to an agency's reputation to take on a bad client).

If anyone wants to email me I'd be happy to go into more detail privately. If you have any general PR questions that aren't agency-specific (hiring agencies, hiring consultants, how PR can be successful, etc), I'd be happy to answer those publicly.

Edit: In case anyone is curious, other YC companies SG has worked with are Xobni (I worked on this account), Scribd (I worked on this account too), Loopt, Zumodrive and maybe others I'm forgetting.