It may not be necessary to make more advanced terraforming cost less PP. Just being able to do it (much) faster is probably enough incentive to get the more expensive techs.
In your proposed new techs, do the later terraforming techs require the earlier ones?
The tech tree scripting should be trivial - just copy an existing theory tech or application that does nothing (there are a few) and change the stringtable entries and cost. Since there won't be anything to unlock and the techs won't have any direct effect, there's no complicated effects scripting to be done.
To make it possible to build a terraforming project, we'd need to add the capacity to add "actions" or "projects" or somesuch to the production queue. These would cost PP and take time to build, and when complete, would cause an effect to fire, probably for one turn. That effect would change the planet type (environment) in the case of terraforming projects.
Alternatively, something like the above could be hacked with the current system by making a building that changes its planet's type once, then does nothing in subsequent turns. I forget if there's a destroy effect, but if so, the building could destory itself after firing the terraforming effect.
(Aside: we might also want "ongoing projects" or somesuch, which cost PP each turn, but which also have effects that fire every turn...)
Also, FYI I've made (almost?) every tech cost 1 PP and take 1 turn to research in SVN. There's not much point in maintaining a pretense of balance at this point, especially when it comes at the cost of making things much slower to test.
Also also, there are ships in the tech tree now. You could make a tech that unlocks a nominal terraforming ship part, although it wouldn't yet do anything. See the techs in techs.txt that have names starting with SHP_ Unlocking is done in a manner resembling
unlock = Item type = ShipPart name = "CO_SUSPEND_ANIM_POD"