How about this.
£100 a go.
Yes £100 a go. I will have a word with the suppliers so that the machines actually take the money.
£500 jackpot.
This is probably not legal, but let's carry on.
Each £100 adds to the cashpot. Once it reaches £500 a reserve cashpot builds in the background.
Actually let's not get into the semantics as this is clearly illegal, but that aside

Would any of you quizzers pay £100 stake for a game that you could win £500 on and lasts for approx 5 minutes regardless of skill?