I think the main 'limiting factors' for Plant based proteins is the low levels of Leucine (good old mTOR ) and lysine.
As long as these are addressed, then I can't see any 'non-biased' reason for animal based proteins being better than Plant.
IIRC Soy Isolate is high in all essential Aminos' with the exception of Leusine...might be difficult to get a vegan friendly Leucine!? Maybe @Superfreak can shed some light?
Now I have had a chance to research a bit and cost it out, I am pretty sure a very nutrient-dense, protein-adequate vegan diet would cost less than a typical 300/500/70-type meat-based macro diet.