But think the cost should be higher. More like
1000 stone
300 wood
50 iron
50 pikemen
100 workers
200 peasants
and take 18,000 man days to build.
Another option is something smaller, say, a town.
only 1 rare trade good bought
only 1 rare trade good sold
only 20-40 pikemen
.5 on the civ level
add 50 to tax base.
which may (or may not ) be improved to a city.