Halion is available for around $350, Native Instruments' Kontakt for around $400 ,and MOTU MachFive for around $260. For Cakewalk to spend the effort required to create a plug-in up the standards these instruments have set would increase SONAR's price dramatically. The beauty of plug-ins is that you have a choice on how to accessorize your core DAW.
That said, there are expansion packs for Dim Pro and even though Kontakt is my "main squeeze" for samplers, don't overlook what Dim Pro can do. I have many tracks where Dim Pro came through when others didn't.