The underlying problem here is that the cost of making an SoC is enormous. So to make, say, an ARM desktop class machine you have only a few choices:
1) Make your own SoC designed for the purpose. You can tailor it to meet your requirements precisely, but given the low volumes you'll be selling, your system will be at least $10,000 a box, likely more.
2) Use a designed-for-mobile SoC. This will hit your $200-$300 price point pretty easily (perhaps even sailing under it by a big margin), but the IO options will be bad, because mobile SoCs don't neeed SATA or PCIe. CPU performance is likely to be underwhelming because mobile SoCs are designed to hit a power consumption level that won't drain mobile batteries or make mobile devices overheat. Almost all the cheap devboards you can get today are in this category.
3) More recently we're starting to see server or networking SoCs, which gives you an option a bit like 2 but with different cost and expansion tradeoffs. Generally a bit more expensive than 2 because networking won't hit mobile volumes, but better I/O capability. CPU power may still be less than you might like. Examples here are the Macchiatobin board and the dev box that Socionext announced last week.
I don't think anybody disagrees that a proper desktop class machine for these architectures would be great; but there are huge economic barriers to getting there. Personally if I was looking at getting a new ARM setup I'd try something in class 2, likely the Socionext box when it becomes available (end of the year, I think they said).