Alice Banion sings the famous cowboy ballad "The Streets of Laredo". This song was apparently written early in the twentieth century (actual authorship is disputed) and was therefore after the show's time period.
Hannibal quoted the price of gold at $21 per ounce, and $333 per pound. But gold is measured in *troy* ounces, and there are 12 troy ounces in a troy pound - not 16. So the per-pound price is actually $252. And even if there were 16 ounces of gold per pound, $21 X 16 = $336 not $333.