Cost for you to build this system today:

16 X6500s at $550 each: $8800
Power supply (I used this one): $65
4 120 mm fans at $15 each: $60
Various Molex splitters and adapters: $20
16 USB cables: $50
USB hubs: $30

Total: $9025 / 4.2 GH/s or 2.12 $/MH

A lot of those extra things you probably have sitting around already, or you could find better deals.

The next price break above that is at 25 units. You could still run that many boards off of a single supply (400W) so your overhead cost would only be slightly higher.

25 X6500s at $535 each: $13,375
Power supply: $65
Fans: $90
Various Molex splitters and adapters: $30
25 USB cables: $75
USB hubs: $40

Total: $13,675 / 6.6 GH/s or 2.05 $/MH

You will also need a host computer to run the system, but you most likely already have that and the performance requirements on that computer are quite low.

Note that those $/MH are based on 266 MH/s for each X6500. I have every reason to believe that we will reach higher hashrates with improvements to the software and firmware in the future.