How thick a gold finish on a pcb for wire bonding?
According to documents I have from a PCB vendor, a typical spec for wire-bondable electrolytic soft gold is and 1.97 \$\mu\$in (min) gold over 188 \$\mu\$in (min) nickel. These are called out as IPC minimum values, though I don't have the IPC documents in front of me.

ENEPIG (electroless nickel, electroless palladium, immersion gold) plating can also produce a wire-bondable surface. A typical spec for that is 197 \$\mu\$in nickel, 12 \$\mu\$in palladium and 1.1 \$\mu\$in gold. Again these are all minimum values.

Recently I had success with a design that spec'ed the gold thickness as 25-30 \$\mu\$in, but really you don't want a very thick gold layer because excessive gold does bad things to solder joint reliability.