[Wazowskiscar]

If I remember correctly a business class seat was around 60-80 quid each to book when I last did it a couple of years ago.

Economy (I almost booked for mine to America but never did thankfully otherwise that would have been more money to claw back) - Exit rows started as £85 to book ( and rose to £100). Front section of economy cost more, I believe some seats were around £15 and some were more... memory is failing though! For the sake of comfort tbh it’s just part of the price really. Business you can’t go too wrong (I prefer the window but would want to be stuck in that middle pair with a stranger. Also the window requires a bit of a hop over someone’s legs to get out unless your at the back row)
Economy though, definitely pay, most do, keep an eye on the seat map in Manage my Booking and if it starts to fill up rapidly, book!