ASICMINER mine themselves, and with some partners. They don't sell "retail". There is no fixed costs to "build rigs", it depends on many factors.
I beg to differ, there is a minimum cost you can manufacture an asic based miner for (in volume), and it's easy enough to calculate if you have commercial design experience or are willing to do a reasonable amount of research work on the net. Granted, some companies have more 'overheads' than others, but that's just greed or poor management. I'm talking about a well run company with experienced people and modern manufacturing methods.
We're also seeing a very rapid development cycle on the ASICs that is quickly catching up with current top-end chip fabrication technology. That is, we'll soon be seeing hashing-speed advances slow down to the pace of Moore's Law, at best (short of just packing more and more cores into a box).
Combine that with the exponential growth of the entire network's hashing speed we've been seeing lately, plus the difficulty increases that result from it, and it may reach a point where none of the hardware can be sold at a price that is sufficient to justify its manufacture. This can be staved off for a while if the value of BTC continues to climb, but we're facing a situation of diminishing returns. More people are competing to get BTC, and the minting payouts will continue to halve every few years.
So I think the question is
when, and not
if, we'll see mining for the sake of profit become useless. Before that happens though, all hardware has to be bought with the expectation that its worth will diminish rapidly, and the electrical cost may soon overshadow its earning rate.