It would seem like the conversion rate is roughly speaking 1 plunder = 2,7 BP.
This as fixing a ruined castle was 27 BP, now 10 plunder.
Building a pier at 16 BP at 5. 6 would be closer.. but ohwell.
Also never was a big fan of 250 peasants feeding either 250 peasants (and themselves) or themselves and 9000 citydwellers. Thus a city district is better being 250 people when fully filled and also those 250m (750') a side.
Besides, no city makes any sense with that 1 mile and 9000 inhabitants rule.
Tazleford would not have a pop of 250, but 4500+
Similar "king and queen" of korvosa are kings of 2 citydistricts.
Absalom is hardly a barony at 33 districts. (instead of being a empire in its own right)
I guess with both willing allies capable of providing labour and a ready crew or more... one can rewrite that 1 building a month rule for small beginning ministates. Kinda made sense if all you had was a few reformed bandits and maybe some friendly kobolds doing the nightshift.
Now a crew of 100+ is sort of expected.
Plus any slaves captured, the allies and whatnot.