It’s a bit more complicated than that (& apparently I really wasn’t clear before).
So I tested 4 different passives at various levels of the Paladin passive tree. Light of Rahyeh (right at the top, requires 45 points in Paladin & goes up to 12 points at cap), Holy Precision (requires 30 points in Paladin & goes up to 10 points at cap), Faith Armour (@ 25 points in Paladin & goes up to 8 points) and Prayer (@ 20 points in Paladin & goes up to 10 points at cap).
Respeccing points in Light of Rahyeh became cheaper ny 280 gold for each point respec’d, so if you have 12 points in the passive the first point respeced (ie, going down from 12 points to 11) will cost 3,360 gold, the next point respeced will cost 3,080 (going down to 10 points in the skill) & the cost reduces by 280 gold each point.
Holy Precision works the same but will only cost 240 gold per point already invested to respec. So if you’re respecing from the cap of 10 points, the first point will cost 2,400 taking you down to 9 points, then the next respec will cost 2,220, then 1,980, etc.
Faith Armour costs 200 gold per point invested to respec, so respeccing from the full 8 points will cost 1,600 for the first point (taking you down to 7), respeccing down to 6 points will then cost 1,400, then 1,200 for the next point, etc.
Prayer, you guessed it, 180 gold per point already invested. So respeccing from the cap of 10 will cost 1,800 for the first point (taking you down to 9 points), then 1,620 to respec from 9 points to 8, then 1,440, etc.
What I wasn’t aware of (thanks for getting me to have a look into it), was that the per-point respec costs increase at 20 gold per tier of the skill. So the first tier of passives (the ones at the bottom requiring 0 points in a mastery) will cost 100 gold per point invested so respeccing down from 8 to 7 will cost 800 gold, going from 7 to 6 will cost 700 gold, etc.
The next tier of passives (requiring 5 points in the mastery tree) will cost 120 gold per point invested to respec, so respecing from 6 points to 5 will cost 720, going from 5 to 4 will cost 600, etc
Base classes, however, cost somewhat less. A passive that requires 20 points in the base class costs 50 gold to respec per point invested & the base class respec costs increase at only 10 gold per tier. So a passive that requires 0 point in the base class will only cost 10 gold per point invested, a passive that requires 5 points in the base class require 20 gold per point invested, etc.
So as it turns out, reality was a mix of what we thought.