Ajax call always returns 0

Could you place the action (ajaxConversion) in your Data and check?

jQuery.ajax({
  type:"POST",
  url: ajaxurl,
  data: {
      action: "ajaxConversion",
      amount: amountToConvert
  },
  success:function(data){
  alert(data);
  },
  error: function(errorThrown){
      alert(errorThrown);
  } 

});