Here is an old calculation I made:
Ra priest empowerment calculations:
Ra priests empower at 15%. Populations cost = 2
Ra Pharaoh empowers at 24% (in parenthesis). Populations cost = 0
6 x 1.15 = 6.9 (7.44)
7 x 1.15 = 8.05 (8.68)
8 x 1.15 = 9.2 (9.92)
The brake point for making it like 1 more villager is when it empowers 6-7 workers.
But before pop limit time priests boost the eco no matter how many worker you got. Priests can be made from temple so they don’t take up TC time either.
But how about pop effectiveness? The priest has to make up for the 2 pop he costs! Lets find out:
13 x 1.15 = 14.95 (16.12)
14 x 1.15 = 16.1 (17.36)
15 x 1.15 = 17.25 (18.6)
16 x 1.15 = 18.4 (19.84)
17 x 1.15 = 19.55 (21.08)
18 x 1.15 = 20.7 (22.32)
19 x 1.15 = 21.85 (23.56)
20 x 1.15 = 23 (24.8)
Seems Ra priest makes up for the 2 pop cost at 13-14 villagers empowered. But its not a boost anymore. To be a boost it has to be minimum 14 villagers or more that is empowered. If empower 20 vills you finally end up at results equalling 23 workers, that is a gain of one extra worker.
Conclusion?
Early uses of priests to empower are an eco boost no matter how many they empower. When empowering 7+ it makes it + one villager working. But later at pop limit they have to empower at least 14 or more villagers to make it worthwhile!
This might not be completely accurate as they also speed up building of gather sites/ other buildings and can assist in taking out myth units etc. Also the 15% empower is a guarantied bonus and another villager might ad to more bumping/ walking etc.