Like I said, it would be a sideways transfer. Costa is no where near as good in the 2nd half of the season as he is in the first, whereas Lukaku is at least consistent throughout. He's 24 in which he has room to grow, and he actually WANTS to be here. That is the underlying factor that separates him and Costa. That and the fact Lukaku won't have unpredictable behavior.
This is Lukaku's goal scoring percentage at both West Brom and Everton
Top 4: 12 (14,2%)
5-10: 26 (30,6%)
11-16: 24 (28,2%)
Bottom 4: 23 (27%)
And this is all whilst playing on a team where the players are inferior to ours.