So? The likes of Lindelof are quoted at circa £35-40m anyway, far less proven in a proper league. City have discovered with the likes of Mangala and Otamendi that buying defenders from abroad is expensive and risky; Keane ticks all the boxes and has developed into a solid performer, so under a coach like Mourinho with better teammates and hopefully Champions League exposure, he could go up to the next level.
He's already more measured and reliable positionally, and has much better decision making and fitness than Jones, and has the same positive attributes Smalling does (mobile, aerially strong, robust) with a better footballing brain, technique and ability to distribute properly.
He's a lot better than Bonucci was at the same age, and even Vidic was in his latter-mid twenties when he arrived; he's got all to qualities required to become an elite defender. He just needs the chance, and if United don't give it to him, Chelsea Spurs, Arsenal or maybe even City will.
Harry Maguire is a level below Keane, and the only other young British central defenders worth looking at would be Gibson (who IMO is more limited than MK) and Alfie Mawson (who I really like and would probably compliment Keane well and is a Mourinho defender)