So i did the maths for Goblin Builder

Before starting I'll just say that it's not for everyone, and i totally understand anyone deciding not to spend their gems on for whatever reasons

So I'm currently stuck on th12 and anyone who is or has been on this townhall knows how boring and slow it is to upgrade things on this th with the long upgrade times for buildings and heroes, I've still got mortar, wizard tower, cannons and 10 level of warden + 7 levels of king and 2 levels of queen left so even though the gems are too high I'm willing to spend gems to max my base faster (I've saved around 9500 gems anyways and I'm f2p)

So the event is about 60 days

40gems per day * 60 days = 2400 gems (in total)

For 2400 gems you get 60 days worth of value compared to :

5 book of heroes which would give you at max 40 days of value (max level queen ; 8 days) And let's not even talk about the book of building that's super expensive with gems

With goblin builder in use you would also get 63 hours of reduction per builder potion (if you have 6 builders already) and 54 hours of reduction (if you have 5 builders only)

To finish this like i said i do not think it's for everyone, if you're someone who has 2400 gems saved up and to you progress matters more than cosmetics (skins) then I'd say you could go for this and get 60 days worth of value in 2400 gems