Spidey, being in the industry, I can say that the cost of foil cards is signicantly higher than the cost of a normal card. WotC is taking a huge risk here and they are going to have to sell a fairly substancial amount of this to break even. It it sells well, then yes, it will be a huge source of revenue. The thing to remember about costs is that everyone gets their share and margins (the profit for the middle-man) are typically discussed as percentages rather than numbers. So WotC's cost on this is probably $0.10to $0.20 per card. A lot depends on quantity and I doubt they are printing as much of this as they do a normal set. I don't know on packaging and other overhead. The distributor and the retailer each add a percentage to that, so what the consumer gets isn't the price increased by the additional production cost, but a price that's has a ratio based on the original production cost.
There are 2 things to consider here, though. First, Magic is selling very well right now and has been for the last few sets, so WotC has a bit of money to take a risk at the moment. If it doesn't pay off, they probably won't do it again. If it does, then they will.
Second, and probably more important...I don't know what kind of a return policy WotC impliments, but in most cases, retailers have to be the ones to take the risk as not even the distributors allow returns. That's why one of the best ways to get a retailer to take your product is to offer a good return policy so they aren't the ones taking the risk.
Another secret ...most retailers don't really understand the industry. Yes, they sell singles and run tournaments, but they don't really understand why some card sets sell and others don't. My guess is that many retailers will remember the success of the YuGiOh gold set and figure that this will be exactly the same.
So...WotC will probably do just fine on this, but some retailers will have far more than they need and will wish there was a return policy.