Does the cost of an ENIG-finished PCB depend on the surface area of the exposed gold at a large scale?
Definitely. If you work out the cost in gold at current wholesale prices (about $60/g) for 1u&quot; thick gold layer it's around USD 30 per \$\text m^2\$. Probably costs the manufacturer a bit more than that, plus there's the nickel layer to be applied.
Some manufacturers will ask for an adder if you request more than the pads to be gold plated even in prototype quantities.