Depends entirely on what speed you’ll be driving at, on average, and your vehicle (oddly). At low speeds <40 average your roof rack will make very little difference as the added drag is minimal. Spend lots of time on motorways at or near the limit and you’ll notice a couple of mpg gone roughly on an average sort of car. A family member owns a hilux surf and very little affects its mpg – it sits at 25-26 regardless of whether it is in 2wd or 4wd, whether it has roof racks on or not (with and without canoes – never tried bikes on it), whether it is fully laden internally or not – I can only conclude that the majority of its fuel use is in inherent drivetrain loses! I’m not sure what my 306 estate would get, but its unimportant as I put all my bikes inside it.
Got to say I always preferred my towbar mounted racks, less fuel use, bike hidden from direct blasting of cack into bearing areas at speed, all extents of vehicle visible in mirrors and not having to lug bikes onto the roof!