Audiosurf is one of the first games using steamworks and that is completely free to developers so I would imagine he keeps 100% profits.
Excerpt: Complete Suite of Publishing Tools Available Free of Charge
January 29, 2008 – Valve, creators of best-selling game franchises (such as Half-Life and Counter-Strike) and leading technologies (such as Steam and Source), today announce Steamworks, a complete suite of publishing and development tools – ranging from copy protection to social networking services to server browsing – is now available free of charge to developers and publishers worldwide.
“Developers and publishers are spending more and more time and money cobbling together all the tools and backend systems needed to build and launch a successful title in today’s market,” said Gabe Newell, president of Valve. “Steamworks puts all those tools and systems together in one free package, liberating publishers and developers to concentrate on the game instead of the plumbing.”
link:
http://www.steampowered.com/v/index.php?area=news&id=1423&cc=CA