If one pays 3 resources to double Forest Dragon's power (giving him 6) and then plays the Wild Hunt and chooses to sacrifice him, does this count as 6 developments or 3?

I know (or at least think) that in a smiliar scenario with, say - Troll Slayers in a battlefield already with 2 developments - that the Troll Slayer's power is just 1, not 3.

I guess I was wondering if there is just a difference between "gains X power" and "double its power"

I think here should be used printed power.

Troll Slayers power is 3 with 2 devs in bf. I think Wild Hunt counts only printed power because the unit goes to the discard before you check it's power and on discard the games doesn't remember any modifications.

Then isn't troll slayers printed power just 1 with wild hunt?

Yes, it it of course. I didn't notice you were refering to Wild Hunt.