It's Y15,000 for any 3 days out of 10 in the JR East area.
http://www.jreast.co.jp/e/eastpass_sp/index.html
The regular flexible JR East pass is Y20,000 for 4 days within a month. This pass has the same cost per day and a shorter maximum validity period. So there might be some special circumstances for which this pass is useful but not too many -- JR East has simply raised the price (originally Y10,000, then Y13,000) too much, especially since there is still no inflation in Japan.