It can cost a lot to run a GPU, especially at full load. The 4090 stock pulls 500 watts of power under full load[0], which is 12 kWh/day or just under 4380 kWh a year, or over $450 in a year assuming $0.10-$0.11/kWh for average residential rates. The only variable is whether or not training requires the same power draw as hitting it with furmark.