Let's go for a ride.
When building a Zipline, two zipline poles will be placed, one after the other, and then a cable will be strung between those two poles.
As a Decoration, constructing additional Ziplines will increase the cost by the base cost up to the fourth copy, after which all buildings will cost the same. However, unlike the other production and decoration buildings, the Zipline is built in pairs, so placing a "second" zipline counts as and costs the same as the "first" one built.
Every two Zipline poles built increases in cost up to the third pair of poles.
Single Pole Costs
|Copy||Wool Thread||Marble||Metal Rope|
|First (Base copy)||4||2||1|
|Fifth & up||16||8||4|
Paired Pole Costs
|Pair Copy||Wool Thread||Marble||Metal Rope|
|Third & up||32||16||8|