In my game, the players recently helped an army capture a Class 1 Market city.
Do they get XP for that, and if so, how much?
As the campaign was switched to ACKS, other then the fact that the city is one of the biggest in the country, I haven’t ACKS-ified it and done any math to figure out how much it cost to build or anything (if that is required). What would a Size 1 market be worth (if that is important)?
Well according to core book page 135 a metropolis requires at least 2.5 million gold invested.
How much have they pulled out in loot?
How much actual treasure did the PCs get as their part of looting the place? They get that XP.
Then, any income they get from the conquered city later on is considered part of their domain and mercantile income and compared to the threshold as usual.
I suppose you’d also get XP for the proceeds if you, say, killed the emperor and then sold the empire to the highet bidder. Helping conquer a city, though? It all comes down to the actual profits, either up-front or over time.