How to farm power points in Dragon Age Inquisition?

There are a lot of other free or cheap areas that you can get points. I've only unlocked a few things so far to get to skyhold, but now I am up to over 90 power points from just doing side quests in hinterlands, storm coast, fallow mire and crestwood. Hinterlands is the only area I've cleared of missions, the other areas I just bummed around in briefly. I always look at my map to see where missions are when I get to an area and try to string them together to save travel time (hardly use my mounts).

Haven't unlocked any areas in Orlais besides val royeaux.

Worst case, the "requisition" quests are infinite. I found the storm coast requisitions to be very easy. Each requisition you complete gets you a point and you could farm that to infinity AFAIK.

Besides the Hinterlands, you can also get power points in The Storm Coast, the swamp area, and the desert area in the beginning.

Later in the game there are additional places that you can earn them.

You can buy influence from a merchant in Skyhold. Each "tome" gives you 1 power regardless of the price, but the amount of influence you receive changes depending on the price. This is probably one of the fastest ways to get more power.