I've always found it a bit odd to have core jungle items with different costs and I don't like this change either. It at least made a little more sense before because tanks are lower economy but now we're also making the cost of Runic Echoes different from other carry jungle items like Bloodrazor and Warrior. It seems arbitrary and if AP junglers become too good, they'll likely be balanced around earlier spikes. Thus, they'll be made worse at carrying late game or Runic Echoes will be nerfed to be less slot efficient. A cost decrease will negate the Dark Seal nerf for AP junglers but I'd rather be able to carry better when my team is behind and I'm even/ahead. I'd prefer if the base of Runic Echoes was +5-10 higher (even if it's a smaller buff objectively) rather than this change.
APs suffer from unreliability and reduced sustained damage relative to ADs (matters for objectives/clearing too). This does nothing to make them feel more satisfying late game and it instead makes them spike harder when they're already pretty good. I want higher slot efficiency and late game options for greedier builds. I don't want APs pigeonholed into early/mid game spikes and falling off regardless of style, build or setup.
I hate that feeling of hopelessness just because the game goes late. I want a way to play for late game on this class as well (like the old Dark Harvest allowed). Making me weak late game even if I try to delay my powerspike and itemize for better late means that I am chained to my team's performance and can never solo carry from behind. I don't want to be forced into being a discount 2nd tier mid laner unless I itemize/choose a setup that is meant to fall off.
It feels terrible and I think a large part of tilt and toxicity is exactly because people feel like they cannot solo carry a losing team. That's why I want options to at least have a chance to carry if I get sufficient gold late. I realize that some AP junglers such as Eve and Karthus do scale fairly well but I'd like this to be more about build/runes/style rather than champion pick. For example, late game runes like Conqueror aren't viable on virtually any AP champions and Dark Harvest has been reworked into a pure win-more snowball rune with no fallback pattern (also inherently better on laners and still not that impressive even if you're ahead enough to be capable of getting a lot of stacks).
BTW: Maybe junglers could have a jungle item upgrade option that unlocks at 16 for 1500g (something prohibitive so it's only worth it at full build). The point would be to make jungle items competitive with those of laners late game. Selling a jungle item isn't really realistic because many champions depend on Talisman for mana regen and also need the special smites for their kits to work properly. Without those, their Smite summoner spell becomes gimped.