Just taking out Dropbox and Airbnb, the average for the rest already drops quite a bit. With Dropbox at more than $4bil [1] and Airbnb at $2.5bil [2], that leaves the remaining 283 at about $5bil total, or $17.6mil each. Taking out others like Weebly would drop it further.
[1] - http://www.forbes.com/sites/quora/2013/02/07/why-is-dropbox-...
[2] - http://www.bloomberg.com/news/2012-10-19/airbnb-said-to-be-r...