According to the calculator, the most expensive animal is - by virtue of its longevity - the tortoise (£27,787 over 75 years), followed by a small dog (£19,224 over 15 years), a large dog (£17,234 over 11.5 years) and a medium-sized dog (£16,828 over 11.5 years). A cat comes in at £12,322 over 16 years. A chicken will set you back £4,701 (it's not clear whether they've offset the money you'll save in eggs). The cheapest is a fish, at £914.
PS: Whilst you're here, take a moment to see our latest job opportunities for vets.