If you use balanced or at least ground interleaved connections in the ribbon, the RFI should be minimised.
And you can get ferrite clamps for ribbon cable to reduce it further.
I think the price of screened types is a result of it not being used much, rather than the other way around. There is not enough market for real competition, unlike with basic ribbon.
"Twist and flat" type is an intermediate compromise that can also reduce emissions, as long as it's used properly.
That is far cheaper than screened, though not as cheap as unscreened. If you eg. get 50 way and split it down to five runs of ten way, it makes it about £2 per metre.
Note that cable screening alone will not help much anyway, if there is more than one possible current path between units.
Each cable must have its own good return paths, then adding a ferrite sleeve restricts unbalanced currents via different paths and so radiation.