So lets consider JavaScript language.

When I clarify my intent as basePrice being const, why should I be re-calcualting basePrice again and again unless the quantity is going to change(which I hope not).

 calculateTotal() {
  const basePrice = basePrice();
  if (basePrice > 1000) {
    return basePrice * 0.95;
  }
  else {
    return basePrice * 0.98;
  }
}

basePrice() {
  return quantity * itemPrice;
}