I am building a poltergeist, so I would like for it to be able to manipulate things as desolidified, as it is is normal state (it as costs no end, allways on and inherent). Now is the problem : it says that I mus buy the "Affect physical world" as an advantage on his strength. What if I want to keep the 10 points base strength ? Does it cost 0 ? Or do I need to consider that I have to buy the advantage for those 10 base points, hence a cost of 20 ? And I can extend the question if I want to buy more Strength points : do I need to buy the advantage only on the point I buy, or on the 10 base points too ?
I have only the Champions complete book, no other book of the heroe system, so sorry if the answer is somewhere. And I couldn't find it in this section.