I think Hidden.org doesn't have a specified end-user value, since it's too much generic and the extension doesn't fit with commercial purposes.
However, it looks very desiderable by some resellers and could flip it for mid-high $xx.
Great name. One of the top keyword for proxy site. Easy to remember for people to access from school or office. Developed site (depends on your contents) low $xxx to $xxxx, undeveloped low $xx to mid $xx.
Hidden.com sold at an auction last year for $15k, hidden.org would have diminished value - probably high $xxx to an end user and low $xxx to a domainer.