Spend what he generated from sales and competitions. It's not clear if "what he generated" includes the money we're getting from the Premier League, or just from the Championship. He's got £30m in his pocket thanks to last Summer's deals. I reckon we'll make another £30m+ this summer by selling the likes of Saivet, De Jong, Hanley, Murphy etc. If we also sell players around the 1st team like Mitrovic, in this market we could get £20m for him alone.
fwiw I'm not saying Mitrovic is the best striker ever, or that his performances would put him in that bracket in previous windows, just that the market is bonkers so what we think a player is "worth" is probably way below his market value.