I've done a similar math dozens of times and always concluded that the only weapon which is worth to upgrade tier is the BiS weapon. Every other weapon upgrade is too expensive and it's better to just buy the other weapon tier 0 which has a better rank. However, to do bestiary of weak creatures, I've upgraded my class 1 simple bow to tier 1 (only 25k). But I still think its not worth since I cant use mana imbuements and those Fatal triggers very rarely (and makes no difference because weak creatures dies in 1 hit anyways).