After that there are only minor increases. Well, first of all, I would never put more than 24 gigs into a single cache. As for doing freshness checks, our product only performs a freshness check when an certain object is actually requested by a client. So, if an object makes it into the cache and is never requested again, it will never be checked for freshness. It will just be quietly discarded at some point in the future.
Disk space is cheap.
RAM is cheap. Might as well maximize your bandwidth savings. I can see this maybe being a concern if you buy those expensive cache products. Buying a second CE is an expensive proposition.