It's hard to find the right cost-value-age-quality balance when signing a player specifically as back-up. I imagine Raheem is the third choice central striker in a crisis.
None of the Premier League top 5 have great striking depth: the fitness and form respectively of Costa and Falcao at Chelsea is dubious, Arsenal only have Welbeck and Giroud, United have an ageing Rooney and Hernandez, who's not been a worthwhile player since the turn of the decade, and Liverpool have Benteke, Ings and Sturridge, with only Benteke likely to play more than 20 games for then this season. Then there's the likes of Walcott and Remy, who are more hybrid wide forwards who will only play there on a needs-must basis.
Bony is widely underrated on here; as back-up, hes superb, and offers a lot more overall than Dzeko ever did. We've been spoilt in recent times, generally carrying 4 £20m+ strikers in our squad, but our system has changed and such a policy wouldn't be sustainable with only one regular starting slot up top.