Well, we can rely on overclockers.co.uk to leak out sensitie information. X58 mainboard prices have been posted on that e-tailers website. The cheapest board is the Intel reference it'll cost you €303/$413. It only has four memory slots, so you're limited to using three memory modules. The Asus P6T Deluxe isn't that much more expensive at €326/$444 and it has a much better design with six memory slots, it's likely that Asus will have this board certified for software SLI.
The OC Palm edition doesn't look like it'll be a huge seller in our books, as you're paying an almost £30 premium for an external LCD display which plugs into a USB port, that is meant to help you overclock the board. At £281.99 (€365/$496) this board is one of the most expensive out there.
Taken into consideration that the Asus Rampage II Extreme hasn't had its official price revealed as yet, that €400 price point mentioned doesn't seem all that far off now and we can't imagine who will be buying these motherboards. We've yet to see pricing from a wide range of other companies such as Gigabyte, MSI, Foxconn and ECS to mention a few, so there might still be room for some more affordable boards to turn up. It's also worth noting that OcUK isn't the cheapest places to get your motherboard, even in the UK [via OC-UK via fud zilla].