Here is my (preliminary) plan to seed rare trade routes:
For each long-haul tradegood,
Choose two cities (source, consumer) at least 15 provinces apart
Choose n=0-1 cities near to producer
Choose m=0-1 cities near to consumer
Set qty = 100 + (n-1)*25
Each source city sells qty/n +- 10%
Each consumer city buys qty/m +- 10%
set distance = los(source, consumer)
set profit = (125.5 + distance) * distance
set premium = profit/qty
set source city's cost at base price for item (base price is abitrary)
set consumer city's cost at base+premium
set prices for other source cities 1-5% lower
set prices for other consumer cities 1-5% higher
"Near" means, perhaps, within 7 provinces.
Note that quantity available to trade increases a bit if multiple
Longer trade routes are a bit more profitable per unit of distance
than shorter ones.
Ideally there would be enough unique long-haul tradegoods so that
each region could produce one or two of them. So far I've only
thought of eight long-haul tradegoods.
By fixing qty, we make lighter tradegoods naturally better than
than heavier ones.
Constant (125.5) in profit formula may be a bit low. I was trying
equate "ownership" of the trade route with 1,000 gold/month income.
I ignored costs in going to the near cities, and finding goods for
the return trip, however.
Gatecraft wrecks distance computations. We must be careful not to
let gate jumpers easily pull in an easy $6k/month because some
placement of gates and trade routes coincided.
-- Rich Skrenta <firstname.lastname@example.org>