It's still ultimately a £15m transfer fee, event though its spread over time.
So far from what Karlan Grant has done in an Albion shirt, he is not worth that amount. But he is not alone, Livermore was not worth £11m and he is somewhat better than Burke at £15m.
It does happen that clubs make a mistake with large transfer fees, think the following
Silva, WW £35m
Iwobi, EVE £35m
James, LEE £30m
Joelinton, NU £40m
Ziyech, CHE £45m