Sorting out third party ownership is a delicate affair, but most trusted people I've read are adamant that City paid about 32m pounds -- how much of that Porto kept and how much of it went to SL and 3rd party ownership I can't really figure out.
Either way, the fee is high for a pretty unproven player, although I'm not sure I trust Deschamps ability to spot a defender. Mangala is clearly a physical freak, all kinds of athleticism, seems quick and powerful, but he's not the finished product. I liked when we were linked with Benatia more, but I think the idea may be that Kompany is a very aggressive defender anyway and we may see Mangala taking a deeper position and using his speed and power to clean up.
I think the Luiz deal really exploded the market. If he fetches what he got, then Mangala is so young and seemingly talented that the fee is high but not unreasonable. That said, I think City are taking a real risk here, but I find it hard to complain about anything Txiki has done at City, you have to trust their eye for talent.