It is indeed frustrating that so many otherwise sensible people become self-professed experts when it comes to coins, and refuse to give up their entrenched notions. You can tell dad that with all due respect, he should know he taught you well, and you are exactly right. There is a slim chance this coin has an "S" mint mark (San Francisco) under the eagle, putting it at a slightly higher price, $40 perhaps. But if it's "O" for New Orleans, or absent for Philadelphia, trust in your own research and judgment, $20 is dead-on. Condition is everything; this coin rates only about a very good, with some problems to boot, like the uppercut to her chin and the smoke rings she's blowing. Dad would be right if it were a high-grade coin with little circulation wear; but in this condition, it's nothing to write home about. Tell him he was right about mom, she was worth a million dollars, and they raised you right.