Cache sizes generally increase as ram prices continue to decline. This is why many modern drives have 8MB of cache these days, where 32-128K was the norm when I started building PCs. But even so, don't expect a 256MB buffer when a 256MB stick of ram still costs 35 bucks. They sell complete drives for 60-100 dollars these days, so 256MB of cache is not economically feasible at this time.
Also remember how caches work. When read requests are made, the cache is checked first for the requested data, and if it does not exist in the cache it is read into the cache from the drive. Although the incredibly clever cache management strategies used manage to get the desired data in the cache before it is needed, there is no way to insure that it is always there. The time spent searching the cache for data that is not there can add up, so although we tend to think bigger would always be better this simply isn't the case.
Also bear in mind the cache is there to band-aid the physical speed of the drive mechanism. Although it does a very good job of isolating us from the (relative) slowness of the drive mechanism, if we can build a faster drive it behooves us to do so. Although caching alleviates much of the symptom, anytime technology allows to minimize the cause of the problem we have to take advantage.