Found a nice article on celarning some the confusions behind the CAS arrrays, which can save you with some good amount of money and time..

http://blogs.technet.com/b/exchange/archive/2012/03/23/demystifying-the-cas-array-object-part-1.aspx

http://blogs.technet.com/b/exchange/archive/2012/03/28/demystifying-the-cas-array-object-part-2.aspx