The Divine Statue adds the equivalent of 5 Temple levels to favour production. So if your city has a level 15 Temple and a Divine Statue, it's producing the same as a city with a level 20 Temple and no statue.
favor_replenish_rate_to_god_X = square_root ( total_levels_of_all_temples_to_god_X)
If you have 4 temples lvl 25 to same god, then favor rate will be 10. With a statue in a town, will be 10.25 so much smaller increase. Statues, as temples, tend to become less and less important as you own more temples to the same god. with 40 temples lvl 25 to same god, favor rate will be 31.62, and with 1 statue it will go to 31.70. and so on
I would advise to have statues built at least when you own few towns:
- cost less than 5 levels of temple
- boost favor quite good when few temples with that god exist
- favor is very very very important for power-up your empire
- it can be replaced at anytime fast by demolishing and rebuilding something different (by that i mean tower, the other 2 are quite pointless in my opinion)