Another thing everyone is forgetting here is that the "activate x boosts" quests are made cheaper. And they show up at least as often as the "spend Tavern silver" quests in my experience. So my guess is that overall it's a wash. The increased cost for the one is offset by the reduced cost in the other. Although a player might still get by with spending a similar amount for the "spend Tavern silver" quest even with the reduced boost cost by combining boosts rather than fulfilling it with only one. (Now that would be a workaround.)While it won't break the bank for my city, I agree it is bad design to have the castle reward make it harder to complete quests.
The reduced cost is meant to ease the burden for those that are into tavern boosts, not increase it. So it won't impact those who are already developed and no longer require the tavern, but it absolutely will make a difference for those that are constantly using the tavern outside of quests.
The question in mind then becomes is the offset on reduced cost enough to mitigate the increased cost in quests?
Base costs here: https://en.wiki.forgeofempires.com/index.php?title=Friends_Tavern
My guess is probably not for a while. Would need to sit down and figure out the exact cost reductions and consequences to then work out a estimate of what you'd need to be spending on the regular to offset the quests on each Castle Level