Here is an unfavorable comparison of a protective wall with stone to speed chopping/overflow whipping the Oracle (450

) without marble.
True Cost = 150
Production Multiplier = 1 + 1 + 1 = 3
Base Cost = 150/3 = 50
Excess Hammers = [450 (fast hammers that otherwise go to oracle)]*3(production multiplier) + 150 (cost of walls being covered by "real" production of city) - 150 (True Cost of the walls) = 1350
Base Hammers = 1350/3 = 450
Hammers per turn is unknown but certainly less than 50
Modified Hammers per turn unknown but not relevant because of the above
Overflow Hammers = 50 since 450 > 50
Overflow Gold = 1350 - 150 = 1200
The overflow hammers are just rebating what you put in from the city to increase the overflow gold so we can ignore that as a benefit. Recall that two typical Oracled technologies, Code of Laws and Metal Casting cost 1050

and 1350

respectively.
So, doing the protective walls with stone trick nets you 1200

and a city walls, while speed chopping the Oracle without marble nets you +2

, +8

, and at least 1050

plus instant access to a powerful tech. I guess one nice thing about the 1200

is that you can use it to deficit research several smaller techs rather than one big one. Still, IMHO, this isn't even close. I would want at least another 1200

from the walls trick before I even begin to think thats a fair trade.
Obviously if you don't have stone, or do have marble, or have marble and not stone, there is even more disparity.
This seems like a way to demonstrate that this isn't really an exploit since in the most optimal conditions it is significantly worse than the oracle in the most suboptimal conditions.