Cool   thanks Hood!
So, Iwonder  what costs so mutch..... freq , or I/O ?

I presume you are meaning with regards the CSMIO?
If so then I would say both, 24v I/O will cost more and likely also take up more space due to extra components being required.
 Regarding the frequency, it could   be the difference in processor. I dont know what the M and the S/A use but I would presume there is a difference and that difference has its cost difference as well.
Hood