My opinion is that motherboard capable to hold only Intel 4 core and Xeon 4 cores can't be more than 350-380$ in worse case.
But if people are ready to pay than companies will ask higher price. If small number of people buy this board next year price will back on 350-380$.
Over 400$ worth to pay only board where chipset last much longer on market capable to hold multi core Xeons if customers find for good price after few years and until than to use 6 Core Intels. And upgrade have sense when you with overclocked CPU can't be close new model on default frequency. Than time it's time for CPU Upgrade.
When I say close I mean on 30-35-40%