It should be something like: look at your bottlenecks and utilization, look at your costs, look at (cost effective) ways to reduce or remove those bottlenecks or that utilization. Pick the cheapest place to have a bottleneck. Using SSD at an extra 30$ a month lets you use half the CPU and RAM, saving 60$ a month? Go for it.
This book would _probably_ get you moving in the right direction: http://www.brendangregg.com/sysperfbook.html
It should be something like: look at your bottlenecks and utilization, look at your costs, look at (cost effective) ways to reduce or remove those bottlenecks or that utilization. Pick the cheapest place to have a bottleneck. Using SSD at an extra 30$ a month lets you use half the CPU and RAM, saving 60$ a month? Go for it.