I lowered the price from 400 BTC to 350 BTC.
hdminer is currently the fastest miner for the HD 6990 at 708 Mhash/s at stock 830 MHz clock, compared to Phoenix's 650 Mhash/s, a difference of 9%.
Future reply to the random guy who will reply "but I get 7xx Mhash/s with $MINER when overclocking" -> I said at stock clocks !
The numbers don't work for miners with a single 6990, or even four of them. With 10, it starts to look attractive. If I had 20 6990s, I'd buy without a second thought.
That said, your price severely limits your sales volume to those who are setting up large mining operations. As an example, I would venture to guess that at 35 BTC, you would sell far more than ten times as many copies since it would then be attractive to every AMD GPU user here mining with two or more cards, and to many of them with single cards. I'm sure you've already considered how to price your miner, though I'm not sure you considered it from your potential customers' perspective.