The reports I've read put Sahin between loan fee and wages at roughly £10m. He's on £6m a year at Madrid.
If you want to pay £10m a year for him you'd be better off throwing £20m at Madrid and handing him his £115,000 a week contract for 5 years because that's the equivalent cost. Except that's a much better deal because you could sell him on and recoup some of the cash, and if you get six months of injured player or six months of settling in form, it only increases the overall average cost slightly; while six months of the same in a 1 year deal and you've basically doubled the cost.
I don't agree with Rosicky being paid £80,000 a week, if that is the case, and having one bad deal on our books isn't a valid reason to go and make another.