Have a nice Day
These % usually means additional diamonds or event currency you get for same price, but not sure how it can correlate with the offered building.
If it gave 1 royal restoration spell per day or 1 EE spell +1 RR spell per 48h, then i would buy it for 200%.
With only 1 EE spell and statistics weaker than Mauriel in elementals, it's not worth my 1 daily payoff.
For 40 zloty (around 10 dollars) i can make sure that i will get 1 strong evolution building up to level 10 if needed, when the time comes (as I made with fire phoenix before i realised that there will be 1 additional artifact 7 days after and 2 additional 3 months after in fellowship adventures)