I would guess that the CERTUS .400 & .450 calibre Big Game Rifles were fairly rare as these were apparently only produced for 5 or 6 years (I have read 1900-1906).
Most commonly found chambered in .450/.400 3 1/4 as well as a much lesser known number of .450 3 1/4 NE. . It would be interesting to be able to research the Cogswell & Harrison records.
The rarity of the .450NE version is likely explained by the period .450 caliber restriction/ban in India & Sudan, India said to have been the principal British large game sporting market at that time.

I have a reprint of the Cogswell & Harrison 1903 catalog and will have to look up and compare prices, the CERTUS was certainly a fraction of the cost of a double rifle.

As to double rifle -vs- single shot -vs- bolt action repeater; cost being one factor, I suppose there could have been those fellows that simply were more inclined towards a bolt action repeater due military training or whatever.
Remember too that approximately that era Rigby had just contracted the exclusive rights to the magnum Mauser 98, so the CERTUS (as different as it was) would have been one of the only bolt action options in a big game calibre.

EDIT: I'm guessing that the .400 in that era would be nearly perfect as a Lion or tiger rifle...

- Mike

ADDED:
Looks like a CERTUS was approximately 1/3 the price of the same calibre C&H double rifle

SPITTLE