Actually, Stonehenge is kind of a bad example: there are lesser standing stones and stone circles all over parts of northern France as well as the British Isles, so in a manner of speaking, who built the biggest one was sort of 'up for grabs'. However, as far as I know there is no proliferation of 'stone circles' of massive blocks anywhere else: there are large stone monuments, but not arranged the same way. That means there is a specific combination of cultural - religious circumstances that 'causes' Stonehenge to be built, in addition to 'gazing at stars' (Astrology Tech) and having stone handy. And, of course, the stone circles and standing stones in Brittany and other continental locations did not crumble away at all: they still litter the landscape, as do Greek temples not quite as impressive as that of Artemis and 'Royal Tombs' not quite as impressive as the Mausoleum or Cheop's Pyramid and its cousins. I think you hit on a Good Point, though: a half-completed Wonder could be 'salvaged; for stone and other materials, which is pretty much what the production return represents now, or we might add a mechanism that 'converts' the Would-Be Wonder into an 'ordinary' Building, Improvement or District. This would be particularly handy in the case of Stonehenge with its Religion emphasis: fail to complete Stonehenge, and instead it 'completes' as a Holy Site: a lesser collection of standing stones that still acts as a focus for religious observances.