You gotta go down on the cost of a MOBO. I'm unwilling to pay more than $120 for a MOBO. If you're serious about mining then you need to cost a whole system against its expected return. As an example, I have a standard configuration for a single computer that costs $960 (power supply, ram, CPU, MOBO, HD, and Video Cards) and produces 700 MH/s. By assembling these things en-mass, I'm able to build a super computer (in my living room) that kicks out MH/s at a rate of (.7MH/s)/$. That's the cheapest build I could make (that would still function). Buying a $300 MOBO is crazy. There are better options. Check this out: http://www.newegg.com/Product/Product.aspx?Item=N82E16813157207
,@ $126 you can add three cards and, with risers, you can support six. This is everything you'd need for a bare-bones rig.