It's a high efficiency item so I'll pay 68%.
The wiki says it's 1.12
Help: Adamantite Cuirass
Time is not an issue, slow forgers welcome.
Starting NW: Adamantite Cuirass +0 = $62,714
Final NW: Adamantite Cuirass +51 = $1,060,424
NW Difference: $997,710
At 68% that comes to: $678,443