How much effort goes into designing and laying out a PC motherboard?
For a point of reference developing a reference design for a dual processor server type motherboard typically consumes the efforts of:


Three or four board design engineers who own the design and guide its way through development. Testing and verification of the electrical design is part of this work.
Four signal integrity engineers performing simulations on board stackup and waveform analysis for busses like DDR3, PCI3, USB, SATA, SMBUS, RMII, CHipSet Busses etc.
Two CAD engineers working board layout for a 8 or 10 layer board.
One or two mechanical engineers working on packaging, cabling and other mechanical details including board outline, mount holes and heatsinks. 
One regulatory engineer who owns taking the board through emissions, immunity and safety testing.
One or two manufacturing and test engineers to review the design and assure that it meets the needs for high volume manufacturing. 
One product development manager who owns the project schedule and manages the budget.
One or two BIOS engineers that support adapting a reference BIOS to the specific requirements of the board/system.


Expect the average design to require two re-spins. Through this process expect the board design engineers to be involved for 10 to 16 months. Other disciplines will be involved for lesser amounts of time.

This also assumes that the boards are built by a contract manufacturer that provides support to procure parts. 

Not included here is the team of people that would be involved with validating the design from a performance and system functionality standpoint including the OS compatibility.