We applied through Intrax (http://www.intraxinc.com/global-internships) and the cost for the 18 months was a little over $2k. Considering it also includes medical insurance, I'd say it's a pretty good deal.
The main requirement is the US company needs to have at least 25 full time employees -- not sure if YC qualifies.
http://j1visa.state.gov/participants/how-to-apply/sponsor-se...