var n1 = '$40.24';  
var n2 = '$21.57';  
var regp = /[^0-9.-]+/g;  
  
console.log(parseFloat(n1.replace(regp, '')) +       parseFloat(n2.replace(regp, '')));  
  
console.log(parseFloat(n1.replace(regp, '')) -         parseFloat(n2.replace(regp, '')));  
  
console.log(parseFloat(n1.replace(regp, '')) *         parseFloat(n2.replace(regp, '')));  
  
console.log(parseFloat(n1.replace(regp, '')) /         parseFloat(n2.replace(regp, '')));