When this offer popped up, I had been saving diamonds for some time. My goal was to save enough to acquire expansions for one (or some, or all) of my three cities. I had aroud 14,400 diamonds at that time.
The Triumph of the Tides is an efficient and versatile building. I knew I had to take advantage of this offer right away but those diamonds could only cover the costs for one building. Each of my three cities already had a fully evolved Triumph of the Tides building. When checking my inventory, I noticed that the largest city had two Triumph of the Tides artificats and my two younger cities had three each. I decided I should acquire an additional building for my smaller cities only because my diamond resources were limited and the general situation of the bigger city was much better anyway.
Thus, I bought a base and six artifacts for one city and another base and two artifacts for the other one and now there are two fully evolved Triumph of the Tides in a city and one fully evolved building and a level 6 one in the other. A level 6 Triumph of the Tides is productive enough to be worth planting. My cities are both fighting fortresses and catering economies, and they can profit from the versatility of this great building.
Although I spent most of the diamonds which I had been saving to get new expansions, buying the Triumph of the Tides was a smart move because it was going to enable the optimization of the space that I already had. I can't wait to reorganize the space after the incoming FA.