There is more to the cost of shooting than that, though. There was a time when location shooting on video was all but impossible – video cameras were huge (and effectively immobile except on a smooth studio floor), and required a large lorry full of heavy and expensive equipment a convenient cable’s-length away to function. They were also much more restricted in the levels of light that they could work in, so that meant another truck full of lights, with riggers, operators, gaffers etc. And, of course, a handy supply of electricity to power all of this – which probably meant a generator truck, and the operators thereof.
16mm cameras, on the other hand, were light, easily man-portable jobbies, that could shoot in natural light if necessary, and run off batteries – you could fit a director, cameraman, soundman and all their equipment in the back of a Landrover.
Video technology didn’t really catch up to film in terms of quality and convenience until the late 1970s or early 1980s, but by then the BBC had been using 16mm for location shooting for 30 years or so and had a large investment in terms of equipment, infrastructure and trained staff.
And despite what the OP may think, the Beeb has never been what you might call flush with cash – even when the technology was available, it took some time for it to filter through the entire organisation.