What's the differences in cost between server with mainly hdd VS ram storage? Ie with vultur, hertzer etc I though the issue with ram was the much higher costs

What's the differences in cost between server with mainly hdd VS ram storage? Ie with vultur, hertzer etc I though the issue with ram was the much higher costs. Also, could you put all of postgres dB in ram, like with redis? Thus no need for separate redis server since your entire dB is fast in ram.