This hackathon is part of Dreamforce (http://www.salesforce.com/dreamforce/DF13/), Salesforce's big annual user / partner conference. Dreamforce normally costs $1199 to attend, but that was way to big of a cost barrier for a hackathon, so they created a $99 pass that doesn't include sessions or the concert, but does provide access to the developer area (all of Moscone West) and keynotes (Sheryl Sandberg is a headliner).

I'd ask yourself not if $99 is worth it for the hackathon, but if it is worth it to attend Dreamforce as a developer. I have found it to be a useful view into the world of enterprise software-as-a-service, both in what solutions are and are not available, but also in the way they are marketed to businesses. You might even get an idea for your next startup...

And if $99 is too expensive, there is also a free pass that provides access to the keynotes and expo floor only. But not the developer area where the hackathon is located.

Full disclosure: I work for Heroku, a Salesforce company