I think theres one in every 2-3 boxes.
i think the rarety of cards for the heatgold soulsilver set is something like:
every 6 packs= 1 prime
every 8 packs= 1 legend half
every 36 packs= 1 reverse holo red gyrados
every 72 packs= 1 holo red gyrados
every 108 packs= 1 alph lithograph
I think this is correct but with reverse holo and holo gyarados flipped - reverse holo is selling for more on trollandtoad and proffessor-oak, which is odd for a normal holo at best.
Here is what i pulled in my case.
Alph Litograph 4
RH Red Gyarados 4
NORMAL HOLO Red Gyarados 7
Here is what i pulled in my case.
Alph Litograph 4
RH Red Gyarados 4
NORMAL HOLO Red Gyarados 7