Microsoft will start selling the cheaper of its Surface tablet computers on Windows 8 launch day, 26 October, the company has said after sneaking the information into its annual report to the Securities and Exchange Commission (SEC).

The version that will go on sale on this date is the ARM-based Windows RT Surface, with the more powerful Core i5 Ivy Bridge-based version about three months after that.

"The next version of our operating system, Windows 8, will be generally available on October 26, 2012. At that time, we will begin selling the Surface, a series of Microsoft-designed and manufactured hardware devices," to quote the relevant section of the filing that caught everyone's eye.

Exactly what the cheapest model will cost is unknown, with reports putting it as anywhere from £400 (approx. $600) to a £650 price tag noticed on a Swedish website that horrified journalists when it came to light last week. The cost of the add-on keypad is probably on top of that.

The higher of the two numbers sounds unlikely for a device designed to kickstart the ARM/Windows 8 market against Apple and Android competitors that cost around £400 or less.

It is also possible that Microsoft sees Windows 8 as competing against Windows 7 laptops and Ultrabooks which cost more and plans to base prices on the internal computing power not on the class of device.

Techworld's review noted the Surface's use of optical bonding to improve the display; that hints at a much higher price for some models.

On the Windows 8 theme, Microsoft has also announced its new line of keyboads and mice designed for Windows RT tablets other than those made by itself. The The Wedge Mobile Keyboard will cost $79.95 (£52), with the Touch Mouse (which supports touch gestures) costing $69.95.