We buy a 26 year old one season wonder in january for 17, we sell a 24 year old one season wonder in june for 20. Fairly reasonable, I'd say. He hasn't demonstrated anything near what Ibrahimovic, Cavani, Alexis Sanchez or other pricier attackers have done. Anything above 25 m would have been a very big gamble for any club.
Matri did very well for us initially, and so did Quagliarella. Immobile could have done just as well, worse or better. Other, notable strikers who have looked excellent some seasons, reaching a valuation of 15m - 25 m, include Borriello, Osvaldo, Gilardino, Amauri, Floccari and Rolando Bianchi.