Since when J-8 is cheap and has longer leg than FBC-1/JH-7?
J-8 is a cruel joke, not cheap to make (~2/3 of the cost of a brand new J-10 whilst J-10 can beat the crap out of this thing easily, actually, even FBC-1 can beat the crap out of this thing easily based on the results of the internal military excerises, thats why nowadays China navy aviation use FBC-1, a fighter-bomber, instead of their J-8, a supposed interceptor, for intercepting missions.
The only reason that SAC still get orders of the crap J-8 is for welfare purpose to prevent SAC from bankruptcy.