Like the answer to almost everything in IT, it depends. CAS is much more affordable than litigation. So, if you're in an industry that is regulated, you have to balance your potential litigation costs against the CAS expenditure.
There are entry-level systems in the market, like Nexsan's Assureon product, which is targeted at SMBs, that offer advanced CAS capabilities. So, it is a little cheaper to get in these days. And the inherent utilization gains in CAS from single-instance storage are on the plus side because your are storing less data as a result of eliminating duplications.
Other things to consider are that you're eliminating the overhead associated with handling tracking, transporting and storing tapes and the potential impact of losing even a single tape. They all need to be balanced with the initial cost outlay for a CAS system.
You could save a little money and go with a locked network-attached storage (NAS) system, but you'd have to invest in dedupe software and you wouldn't have the robust metadata that CAS offers.
There are all kinds of things you need to think about. You need to think about the gains from reducing your backup window by archiving onto an active archiving platform. And the gains that you get from moving less active data from expensive primary storage to a secondary CAS storage tier that leverages commodity hardware.
So, there are a lot of things to look at on top of the price of the CAS tier. And it's the overall risk and economics of the storage environment as a whole that needs to be considered when you're looking at whether or not it's affordable.
Check out the entire CAS FAQ.