Milan aren't really against signing older players if you've noticed. Keita would be a decent player for them, I'd say we'd get 7-8mil worth of valuation for him as well tho. 15mil cash + Keita (7mil)= 22mil. I happen to really like Afellay, he looked like shit the last 2 games but thats b/c he just came back from a bad injury and the games don't matter. He was very decent for us last season and he was our best player in the pre-season (along with Thiago). We got him for 4mil (maybe 3.5?) and I think that was a steal, we'd have no problem getting 6mil for him (prob 7-8 valuation worth in swap).
I think it'd be a good deal for Milan, and we'd get a WC defender with about 4 years left of prime (28-32, for a defender).