I wonder if/when we'll reach the point that it's cheaper to manufacture SRAM (with 6 transistors per bit if I recall correctly) than it is to manufacture DRAM. (with 1 transistor and 1 capacitor per bit)

The transistors get smaller every year. The capacitors, like you say, don't anymore. At some point those 5 extra transistors will be cheaper than the capacitor, unless Moore's Law well and truly bites it.