PerfectPhase basically told you why. "For cheap" is not all that possible for a WP7.8 device, especially if you don't want a carrier and data plan contract tied to it.
I think it would be very smart of Microsoft to create such a device (again). More importantly, it would make sense to also license the OS to OEMs to create their own devices. Base it off WP8 and include everything but support for phone hardware in it. Basically equivalent to Apple's iPod Touch. This may be a declining market, but it's still a significant market, and one that ties well into the full ecosystem of consumer devices that Microsoft is building.
But, it won't be called Zune. The Zune brand is dead, and there would be nothing worse that Microsoft could do then to try and bring it back now.