You can't use such a strict criteria in football, so many variables.
If you want numbers to match up then no transfer is ever worth doing because we got Pogba for free, Barzagli for 300k and Vidal for a shade over 10M
3.5M for an experienced German international who would be coming on a free + a small signing fee is a bargain if he recovers well. If not, it's not even a big risk.