While querying the web for information about the CAP theorem I found an interesting paper titled “The five-minute rule twenty years later, and how flash memory changes the rules” by Goetz Graefe.
The “five minute rule” is the time interval between accesses the same data for which is does not matter if the data is stored on disk or in memory (cache). Consequently, if the data is accessed more frequently it should be stored in memory (cache) and if data is accessed less frequently it should be stored onto disks
In 1987 the five minute rule was valid for a data sets of 1 KB. In 1997 the five minute rule was found to be valid for page size of 8 KB. In 2007 the five minute rule was valid for a data sets of 64 KB. For these calculations the prices for 1MB of RAM were estimated to 5000$ in 1987, 15$ in 1997 and 0,05$ in 2007.
Another interesting aspect mentioned in the paper is that small data sets can be kept in memory for a very long time now (2007) without having to swap them out of the main memory because the break even point is a very long time interval, longer than the typical time between two snapshots.