£870 for the front and rear bumpers on their own sounds ok.
I got a full aero kit, factory painted and including the additional aero spoiler for £1030 fitted, which from a main dealer was a good buy.
Oh, and definatley get the kit, it will look fantastic, you didn't mention the aero grille though, so unless you already have that too, then I would also order one to complete the look.
Regards,
James.