Aircell is worth every $$!
I have used it under colourbond roof on 2 sheds, and the temp reduction is amazing! When the brickies did the walls underneath a patio structure (to make it a lockable workshop with the aircell), even they were impressed.
I usually buy the retro fit rolls, like you say, not cheap but worth it. Get the aircell brand tape as well to cover the seams.
I am in WA and it is always hot here. Also, any large sheds at work (Pilbara) the walls and roof are aircell lined!!
pic is of 1st shed during construction