If that's the price in the UK these days then I would do a pan drain and fill every two years after the vehicle reaches 60k miles to keep the fluid in good heath.
A standard drain and fill only replaces about 1/3 of all the fluid in the transmission which is why I'd recommend doing that every 20k miles/2 years.
The last time I purchased some Toyota T-IV, which was a number of years ago, it was something like £60 for 5 litres so I'd change it less frequently.