Price? Probably not dissimilar to the upgrade from Cooper to Works - I might be talking out of my bottom, but I'd guess at a new exhaust system, chip plus programming and supercharger belt. not going to be less than £1500, maybe as much as £3000 if a new head is involved.
We don't really know what insurance group the S will be - somewhere between 11 and 15 I'd guess. Although the Works won't be any more expensive to fix, or more easy to steal, or less safe in a crash, if you're _lucky_ they'll add 4 or 5 to the rating. If unlucky, they'll just class it as a modified vehicle and make up the premuim based on capricious whim - and I know from my modified Mini just how much that can hurt the wallet!
MPG is reasonably poor for the S to begin with - but then you won't be buying a Works S to commute or thrash up and down the motorways, will you?
Given the power to weight ratio of the S, I wonder if the Works S is really required. If they load it with other toys, like a LSD, then it miht be more interesting, but I suspect that the S is car enough for the vast majority of us.
Certainly the cost of the Works over the Cooper isn't worth it when you consider the price of a Cooper S - it's only selling because the S isn't available yet.