I'd snap their hands off for 20m, Coates and Kelly can play same as his level if not better in the future.
Skrtel is currently miles and miles ahead of Coates and Kelly. If we sold him for 20 million we would need a replacement for next season and maybe even longer. Could we get a ready made replacement for 20 million? Maybe, maybe not. Not worth it. If he's that good City will be in for him next year with more money, and maybe the youngsters you mention might actually be worth talking about in a first choice CB sense.