I don't agree with your accounting in the slightest.
You are assuming 20 NP's per for the Perform, ignoring the
"spillover", assuming 20 soldiers per province, rather than the 10
peasants you can work it down to (See GIVE and TAKE), and most of all,
ignoring the "ripple effect" of civilization.
Let's see, from the
rules:
feature contribution
------- ------------
safe haven 2
castle 1.5
city 1
tower 1
temple 1
inn 0.5
mine 0.5
So if you build on your mountaintop: a castle, an inn, and 2 towers,
that wilderness mountaintop is now civ-4. Which means all surrounding
provinces are civ-2, which means all provinces surrounding them are
civ-1 (at least). Here's the pattern:
1
121
12421
121
1
So even using Scotts revenue/province figures,
which are low because he used 20 soldiers to garrison, that's an
income of 66+4*46+8*36 = 538 Now upkeep of the noble costs 50g/turn
to HONOR him, so the net is 488g. Contrasted with, shall we say 8
nobles doing perform. Income is 800g. Maintenance of the nobles is
400g, so the net is 400g, less than for the castle.
We are only considering nobles as profit engines and ignoring other
things they could do during the month, but that is true for both
sides. As far as I can tell, you don't have to be in the structure
all month to collect the income. So that noble in the castle can keep
on building temples, being sure to knock off at the end of the month
and eventually she will get to civ-8 or even civ-16.