So Steam has just upped the price of Nightmare in North Point. I'm guessing they had it at the wrong price before now. It's gone from £3.19 to £4.30 ($4.99 to $6.99 in the States). It now matches the console price, so was obviously a mistake.
HOWEVER. Green Man Gaming (the UK site, at least) is still selling the Steam codes for the DLC at the original, cheaper price. Also, their 20% off voucer (GMG25-UAAHK-6AI9S) works on this purchase, so I just picked it up for a ridiculous £2.40. Act fast, save money, bombers!