The difficulty of Spire depends on how advanced your city is; there's an algorithm that looks at your ranking & stuff. I forget which things directly effect the Spire but there are older discussions here on the forum that go into that.
If nothing has changed from the last definitive post that I saw on the topic, the cost in Spire (as well as Tourney) is some constant multiplied by a formula involving:
1. Number of mandatory technologies researched (involved in an exponential function)
2. Number of expansions deployed, with premium expansions contributing less towards costs (by a multiplier)
3. Total number of AW levels (also by a multiplier)
4. The highest relic boost you have (also a multiplier) (There was talk about finding out if Mountain Halls contributes to this, but no data was forthcoming) (Usually ignored as a thing a player can control)
5. The # of the particular encounter you've run into in Spire.
The relevant details are in
this post. I don't know if any further studies on this topic has happened since then.
The one takeaway from it and the supporting post found
here is that the number of expansions you have starts off as the single most important factor in controllable costs, aside from the very obvious (and very undesirable) case of
simply not doing the event.
I feel dirty just saying that last bit.
EDIT: I keep referring to those same two posts whenever this topic comes up, and I half expect someone at least once to tell me off about misinformation and point me to something more recent. I really hope this happens. I don't want to believe that the formula hasn't been changed for about two years at this point.