You can only pillage-repair-pillage a tile that wasn't pillaged at the end of the last turn. So, you really can only keep up 1 pillage/turn indefinitely.
To the OP, it is roughly 15gpt, but unit maintenance costs reduce it (depending on era) to roughly 10-12gpt. In the ideal scenario (adjacent improvements on roads *you didn't build*) a scout can pillage 2 tiles/turn, a horseman 3, a knight 4, but it still takes 4 workers.
So, in the renaissance, 4 tiles = 60gpt - (8gpt for 4 workers) - (2gpt for a knight) = 50gpt.
I typically pick an enemy city to raze, and I build roads on the improvements *while* it's razing down, so that I don't maintenance costs on them afterwards. Otherwise, you're looking at 42gpt after the 2gpt tile maintenance. (Reduced with Commerce of course)
This works better with the Inca, because they pay zero for road maintenance, so you don't have to be as careful about roads.
Two such "pillage farms" net you 100gpt, and they're an excellent way to heal mounted units near the front. I typically capture a city with a horseman... he gets reduced to 1 hp... so I go pillage 3 tiles in one turn and he's back in business.
This has more impact early in the game, 1) because gpt is more scarce, and 2) because unit and tile maintenance is cheaper. So if you can get a city in a peace deal around t60 and raze it, you can get your universities for free. Pillage-repair is so broken.