To be honest with the market the way it currently is to get a proven goalscorer at this level you're going to be paying around £7-8m at the very minimum. Kodjia was mentioned earlier in the thread I would love him here but Bristol City rejected a bid from Villa for him saying they wanted £10-12m. Jordan Rhodes cost Boro £11m, McCormack cost Fulham £11m when they first bought him, Newcastle have just signed Gayle for £10m, Bournemouth signed Afobe in January for £10m. The only exception to the rule seems to be Gary Hooper.
Fact is if we want to sign someone who can score goals at this level, then we're going to have to fork out money. McCormack is as close to a guaranteed goalscorer as you get at this level. 67 goals in the last 3 seasons, at that's playing in teams that have done pretty poorly. Imagine the numbers he could get in a team where he has the likes of Naismith, Hoolahan, Canos, Brady, Olsson, Jarvis, Howson and Jerome supplying him. The way I see it as well, it really doesn't matter about his age. If his goals can fire us to promotion this season then his fee would've been justified, plus he's not a player that particularly relies on pace or strength, so even after this season he could still be a very good goalscorer for 3-4 more seasons.