CPU L2 cache runs on the order of 50-100 GB/s. DVD writing runs, at most, at about 21 MB/s. That's something like 20,000-50,000 times slower than CPU cache.
Even the slowest RAM used in cellphones is capable of about 250 MB/s throughput.
Cost is not an issue. Additional memory is simply not necessary past a megabyte or two because the consequences of interrupting the data flow to the drive are almost none: a few PIEs, PIFs here are there, nothing worth mentioning.