Some relic upgrades are flat out inferior to other upgrades and are never used. For example, Quick Dismiss is favored over Quick Summon since "Dismissing a demon->Summoning a demon" is more common than "Summoning a demon->Dismissing a demon" and can instantly get an ally out of a bad situation.
Moreover, balancing all the relic upgrades so that they are somewhat equal in power is hard and would limit what a relic upgrade is capable of.
I propose that relic upgrades cost varying amounts of relic points to obtain. A relic point is gained every N levels. This would expand the design possibilities for really powerful relic upgrades and adds another layer to strategy to the game. Players could choose not to spend points to save for an expensive upgrade or gain the immediate benefit of a cheaper one.
Furthermore, adding relic points could prevent certain relic upgrades from being too obsolete in the late game. No one picks up Recycle when they're at Tower:15, but if the relic point cost for Recycle decreased with player level, then it could be a viable option if a player needs a quick injection of credits.
Relic points could play a role in negotiations. (Relic points would have to be somewhat common if this were to happen.)