Just roughing it, 2TB P3600 is rated for like ~11pbw, so 11000tbw at 1.2tbw/per plot is 9166 plots that are 100gb plot files which I am rounding to 1 petabytes of plot files. So if 64gg dimms are $300 you need 32 of them or $9,600 worth. There are 2TB P3600s for $500 on ebay right now. So you could buy about 19 of them for cost of 2TB RAM. So, unless you have plans to make more than 19 Petabytes of plots files, you are better off buying the SSD.
This does not consider though that you could resell the RAM but not the SSDs without a steep discount. Still though I don't think RAMdisks make sense unless your ambitions are huge. Also you can have multiple SSDs in one high core count computer to run more plots in parallel on a single system than a RAMdisk and plots per day is the important metric not whether a RAMdisk creates them a little faster,. Also, your cost of the supporting hardware, cpus, chassis, electricity etc... per plot created are much lower with SSDs from being able to stick multiple in each system and running more plots per parallel on the same system than a RAMdisk.
But yeah burning out SSDs sucks but it will probably be a much better return than a bunch of RAMdisk systems.
If I fudged the math I am sure someone will point it out.