Evidence based speculation, page-32

For the smelt temp , guessing you would want to look at the Cu as it has a higher melting point than gold
its melting point is 1085 C (it also has a higher specific heat)
Lets say 1200 degree smelt needed so 1200 less ambient temp of say 20 C = 1180 C
Specific heat Cu = 0.39 j/g * 1180 = 410 j/g add around 2j/g latent heat of fusion = 412j/g
so 412 Watts needed per second to heat 1 gram 1180 c
so 0.4kw per second for 1 gram or 400 kw per second for 1kg
or 0.11kwh for 1 kg of Cu to be melted
say the local power cost 30 cents per kwh then guessing costs 5 cents to melt 1kg of Cu if everything was well insulated .
(Welcome to be corrected on the above ie the J to watt conversion is off the top of my head from school 60 years ago )

Anecdotally Cu has been getting smeltered for a few thousand years so should not be to big an issue with energy costs if its used to recover gold .