In the Conquest game I played, I do a lot of Pop Rush because of the following:
1 pop = 19 shield = 19*4 = 76 gold (if you have 19 sheild left to complete pdtn, then you probably need 76 gold to rush)
1 pop = 10 food (with granary) (Despotism, cities with Pop 6 or below)
10 food = 76 gp therefore 1 food = 7.6 gp
In early game (Ancient age), you probably makes what, 20 gpt ?
Assume you have 5 cities, you can dedicate only 20 / 5 = 4 gpt to each cities.
Further assume the city you would rush to have 2 food surplus per turn, which is 15.2 gpt from this city alone. Compare to 4 gpt for $ rush, Pop rush has faster recovery rate.
To recover 76 gp = 76 / 4 = 19 turns (I use 4 instead of 20 here because I left other gold for rushing other cities)
To recover 1 pop in this city only = 10 / 2 = 5 turns
Of course, you still need to consider unhappiness, lost of commerce for 5 turns, etc.
19 / 5 = 3.8, pop rush is 3.8 times faster with the assumption of 2 fd per turn in one city and 20 gpt for entire nation.
This is another point why granary is useful (therefore Pyramid)
Regards