This has been in the region of £20 recently, just did a search and a lot of places are now listing it as £13 and HMV are selling it for £10.99 delivered.
As always go via Quidco for further savings.
Next best price is £13.43 from the Hut.com
EDIT - All early CSI Boxsets at £9.99.
All recent CSI Boxsets at £10.99.