Some signings are about the rugby field, and some are also about selling the game. Izzy falls into the latter category, whereas Gill goes into the former.
I think Folau has the potential to help lift rugby's popularity again in the lead up to the RWC15. The talk seems to be that he'll sign for two years, which would take him through until then. That's about as "long term" as any prospect can be in this day and age. I'd say that he's worth paying the same amount that Genia receives, even if he is never as influential on the pitch.
As for Gill, he's young. I have no doubt he'll get a top-up next time around. I also doubt that the trade-off you describe exists. My take is that Gill was not topped up because Pocock and Hooper were deemed to be ahead of him at 7. With only 30 or so available, it doesn't make sense to top-up all 3.