With the GTAIV powertrade, don't you only get that 20% bonus if what you are trading in goes towards reserving GTAIV? So yes, in theory you MAY be able to get the 50% (although I doubt it, I think they will limit it to 30% or whichever 20% promotion you choose plus the 10% from the edge which stacks with almost everything), but that 50% would only be towards GTAIV, it's still 30% if you are getting cash or credit (not towards GTAIV) back.