What is the cost to go from London to Highclere castle by taxi, one way?
Take a train from London Paddington to Newbury and take a taxi from there.
Cost of Taxi to Highclere from London?
It's 66 miles one way.
A cab from the center of London to Heathrow can cost $60 + and that's only a 16 mile one way ride.
I'm a math atheist.
You do the math.
I have no idea other than "a great deal of money". The taxi will surely have to travel empty all the way back to London, so I assume you'll be paying for a round-trip.
Unless you want to pay a good £150 plus for a taxi listen to Nancy
A google search for taxi fare finder gave me: https://www.minicabit.com/taxi-fare-calculator
This gives prices between £100 and £125. That is for a pre-booked "minicab", not a taxi from a taxi rank.
https://yourtaximeter.com/ gives an estimate of £239 for a "black taxi", i.e. one from a taxi rank.
Since the castle has a very irregular schedule make sure it is open. A taxi is out of the question.