the current going rate seems to be just over a euro per square foot
it will generally cover you for employing subcontractors/one-man bands/friends/public liability, it's not only relevant for direct labour builds either as you'll need it for anything not covered in a contractor's scope, and you won't be covered by standard buildings insurance
this appears to be a fairly simple product, charged per square foot for two years fixed duration, with no refund for early termination
bear in mind, though, that perceived wisdom is that you need to take this out BEFORE you dig in a spade, we managed to get ours (and hence the two years to start) when we had to start the direct build elements of the project which gives us two years to finish off
having liability cover is often a condition of site purchase, so take legal advice (which this post is NOT)!