Toll House
<iForeignConnectedCommerce>0.5 + 0.1*CitySize</iForeignConnectedCommerce>
<iCitiesRequiredPerForeignConnectedCity>1.0*3</iCitiesRequiredPerForeignConnectedCity>
<iMaxForeignConnections>4*Citysize<iMaxForeignConnections>
<iMaxForeignConnectedCommerce>8</iMaxForeignConnectedCommerce>
Commerce values for Toll House
size 1 = 0.6 (with 3 connected cities of value 1, or if foreign city factors do not count)
size 2 = 1.4 (with 6 connected cities)
size 3 = 3.2 (with 12 connected cities)
size 4 = 4.5 (with 15 connected cities)
size 5 = 6.0 (with 18 connected cities)
size 6 = 8.0 (capped with 24 connected cities)
size 7 = 8.0 (capped with 21 connected cities)
size 8 = 8.0 (capped with 21 connected cities)
size 9 = 8.0 (capped with 18 connected cities)
etc.
As you can see, small cities won't get burst economy by just building a single building, but they need less connections to achieve the lower cap. Bigger cities also need less connections to achieve the cap since they generate commerce by being an attractive trade partner by themselves, which seems realistic enough? The hard numbers are thrown out of my hat, they can be tweaked with the different fixed values and multipliers.