The BBC don't pay the prize money anyway. The show is made by Remarkable TV, which is a subsidiary of Banijay, who make everything from The Crystal Maze and Tipping Point to Peaky Blinders, Grantchester and Wallander. While the BBC, as the commissioning broadcaster, will obviously have a lot of input into the programme, they will simply pay a fixed fee for its production, with Remarkable picking up the bill for any successes achieved by its contestants.
Given the overall costs of creating TV programmes (with costume dramas, for example, coming in at around a million pounds per hour), a few slightly higher prizes are just a drop in the ocean anyway.
The cost of providing free TV licences to all those over 75 years of age would, in just a few year's time, reach a total of one billion pounds per year. That's a hell of a lot of quiz show prizes!