The markup on plastic is phenomenal, as all the expense is in mould setup. That's why marking up the Necrons, long since having paid for their R&D, hurts.
For resin, most of the cost is in the act of moulding a copy.
Metal prices have been worked into model costs for some time, so GW pocketing the difference, instead of doing as Mantic did and including twice as many resin models for no change in price, proves GW are taking the piss.
Hardest hit are the likes of Heresy - all those Ogres and other big guys cost vastly more than they did - the Ogre with Table is now £26, and I remember it was £8
Impact charge appropriately - they're yet to break totally even after 5 year, I believe, but are heading that way.