OoC:
An other approach follows. . . .
Mines [Generic]
DDC: 12.
Build Cost: 10 GB.
Upkeep: 0.25 GB/turn.
Benefits: In a province with Mines, their owner receives a +2 bonus against Contest (Guild & Trade).
In addition, Mines increase province growth by 1 GB per year (applied in spring).
Restrictions: Mines may only be constructed in provinces with hills or mountains (or an equivalent terrain). If primary terrain should be one of these acceptable terrains, then calculate DDC and Build Cost normally. If primary terrain should not be one of these acceptable terrains, then compare the primary terrain with the most favourable acceptable terrain present and calculate DDC and Build Cost based on the least favourable of these two.
Examples: In a plains (clear) province with secondary terrain of hills (rugged) and medium mountains (forbidding), the most favourable acceptable terrain present is hills. Hills are less favourable than plains; so, DDC and Build Cost are calculated based on hills (i.e., rugged terrain). In a swamp (forbidding) province with secondary terrain of hills and medium mountains, the DDC and Build Cost are calculated based on swamp (i.e., forbidding terrain), as swamp is less favourable than hills.
Affluent Mines [Generic]
DDC: 17.
Build Cost: 40 GB.
Upkeep: 0.5 GB/turn.
Benefits: In a province with Affluent Mines, their owner receives a +4 bonus against Contest (Guild & Trade).
In addition, in a province with Affluent Mines, there are available one extra level of guild and one extra level of trade (beyond province level).
Restrictions: Affluent Mines have the same restrictions as Mines.
Note: It is possible to upgrade from Mines to Affluent Mines. However, upon completion of the upgrade, the Mines's bonus to province growth is replaced by the extra holding availability.