I've had to bite the bullet and fork out £35 (inc delivery) for the US NATO -> GA adaptor. Cheapest I could source the individual jacks for was around £7 mark. I think it's an absolute disgrace at that price. I've even had one company try to charge me £5+ for the 1/4" standard audio jack you can get down at Maplins for £1.50 (which is pretty expensive anyway!).
These aviation companies are pretty extortionate. But it's better than forking out ~£60 + ~£40 then who knows how much import tax for David Clarks own adapter. For a piece of wire with 3 plugs on they must think their customers are mugs.