The chance of getting gold foil cards (without scout passes) are 5%. But if you purchase Scout passes for your pack openings, you'll get a boost of around 13% than 5%. It was actually 15% but every 2 weeks, the chance gets decreased by 2% until 10% will be the final chance to get a Gold Foil card with the use of Scout passes.
I've opened 70 packs so far and I have 110 packs staked :)