Do these new ASIC miners really pay for themselves in 5 days?
The Bitforce 50GH/s miner costs $2,500 today, and according to the following calculation it would mine enough coins in about 5 days to pay for itself.
I am sure I am missing something here. Why would they even sell this hardware at all? Surely it makes more sense to keep the hardware and just set it loose on calculating coins and cash them in.
There's a factor called Difficulty, the more hashing power exists on the network the more difficulty exists, therefore it's harder to mine coins. Right now The difficulty factor would let you recover your money fast because almost no one has ASIC chips, when they ship in bulk the difficulty factor will rise to 30-120 million during the year. To know by how much the difficulty will rise it's impossible, so it's your call.
Perhaps they are selling pre-orders and while that's happening using their own hardware to rake in the coins. By the time the hardware ships the difficulty may have racked up making their hardware less valuable but they've double recouped in the meantime (early access to the hardware + sale of the hardware). The upfront investment in these chips must be pretty big though - still quite a risk for the hardware vendors.