Imo it would need to be a little bit more costly than a guild house pack. After thinking it through this is how I've predicted and analyzed it:
If it was the same price as the guild house pack, revenue would only be produced temporarily. After so much time, the people who have previously bought the guild house pack will run out, but players will continue to buy the event house pack because it is simply cooler. Some players won't feel the need to buy the guild house pack anymore since the guild house pack has "gone out of style" while others will prefer the classic yet simple guild house. Ultimately, this will cause what I like to call a consumer spectrum (seriously, I coined the term) which will create a minimum amount of revenue.
Solution:
Since we're basically dealing with consumer psychology, we'll need to find a way to give the players both a reason to buy a guild house pack and a reason to buy an event house pack. Think about it; if we made the normal house pack and the guild house pack the same price, which would they prefer? Exactly, the guild house pack, but as I already mentioned, some players might prefer the classic. With this in mind, our best bet is to make a hierarchy system of houses. Since newer players usually buy a normal house pack before a guild house pack, one could assume that a player would buy a guild house pack before an event house pack. Therefore, we would need to give the effect that an event house pack is better than a guild house pack, and there is no other way of doing that than to make the event house pack cost more than the guild house pack. It's a win/win, because this will give the players a goal of which they can achieve, while it will give your gods more moola.
However, this is pure speculation.