Happened to me, first time so i had no idea how to quantify Kullervo’s strenght too. I only realized when i saw Undercroft enemies being lv140+ lol

I’d say keep it for when you really need it, and you will need some at some point. As others pointed out, inventory slots are the biggest plat consumer, and as you expand your arsenal, you will feel it. Then is catalysts and reactors, which normally aren’t available in large quantities - i’d say buy catalysts but wait and be patient with reactors, as they have a much lower consumption rate and you’ll eventually get all that you need for free.