I went by the ratio from most efficient purchase as it made the math easier. But you are correct that the closest approximations are more expensive. I didn’t realize the soul scales were so different! 10 souls/gem at best, only 5.3 at that range.
I’ve seen it very frequently in PvP. I was surprised bc I assumed most folks at higher levels already had Dragon and Celestial so it seemed unnecessary. But I never did the math on it so I guess this is why. Tempting!
It is very useful armor to have. You don’t have to continually switch back and forth to collect souls and gold. And I felt that the keys you got with it made it very worth the $67 CDN I paid.