However, when I decide if I want to keep a building, because if it is not worth upgrading it has no value to the city, I evaluate value relative to other options and the potential value.
With Ancient Wonders the value of a building becomes variable
Chapter 16 Forbidden Ruins - 41,250 mana/ 3,437.5 per tile
Chapter 17 Forbidden Ruins - 42,900 mana/3,575 per tile
Gain of 1,650 mana/137.5 per tile
Chapter 16 Hungry Treasure - 19,400 mana/1,616 per tile
Chapter 17 Hungry Treasure - 20,000 mana/1,666 per tile
Gain of 600 mana/50 per tile
Chapter 16 Manta Ray - 26,000 mana/2,166 per tile
Chapter 17 Manta Ray - 27,000 mana/2,250 per til
Gain of 1,000 mana/83.3 per tile
When looking at the value of the Forbidden Ruin from Chapter 16 to 17 the gain seems absurdly low
However, when you compare the increase in mana with the Manta Ray and Hungry Treasure - the Forbidden Ruin is the clear choice.
Excluding Ancient Wonder influence - the Manta Ray is a better choice than the Hungry Treasure
However, with the Lighthouse of Good Neighborhood Enchanted and Polished Mana Culture buildings have a significantly higher potential value, but it is dependent on the building being polished by another player.
Looking only at the Chapter 16 Values
My level 24 Lighthouse of Good Neighborhood increases the mana production on Polished Enchanted Mana Culture (excluding bonus productions
A Chapter 16 Forbidden Ruins has a Base Mana Production value of 25,000 mana & a culture value of 8,100.
A Chapter 16 Manta Ray has a Base Mana Production value of 26,000 mana & a Culture value of 2,700.
A Chapter 16 Hungry Treasure has a Base Mana Production value of 19,400 mana & a Culture Value of 6,100
The Forbidden Ruins Potential Mana Value is 45,625 + Bonus for a total Potential Mana Value of 61,875 mana / 5,156.25 mana per tile - a value increase of 1,6718 mana per tile.
The Culture Value of the Forbidden Ruins is high enough that mine is polished for 99.9999% of my collections.
The Hungry Treasure's Potential Mana Value is 35,405 mana/ 2,950 per tile, and the potential of the building being polished when collected is also 99%.
However, with a Culture Value of 2,700 the probability of the Manta Ray being polished is extremely low. With such a low culture value and rarity of it being polished the Potential Mana Value is unreliable making the Hungry Treasure the better choice.
Also, as I level my Lighthouse of Good Neighborhood the Potential Value of the Hungry Treasure will continue to increase - while the Manta Ray might be polished occasionally the value will basically remain 26,000 mana.
Chapter 16 Sorcerer's Feast of the Ruins Set has a total value of 27,250 mana/4,541 per tile - well worth upgrading.
Other Considerations -
The Potential Culture Value of a building - important for the Culture Bonus
As far as the T3 of the Ruins - how many productions and the cost of producing the equivalent in a factory?
As long as this is, it is really a simplification of the variables worth the considering when evaluating the value of buildings.