Manufacturing costs for labour intensive products in the UK are inevitably high - Ribeye have moved production to South Africa for precisely this reason.
The nearest thing to mass production is seen on the continent where companies like Valiant (Portugal) and Zodiac (France) are volume-producing PVC - tubed RIBS.
Some of these products (Valiant / Gemini) are actually cheaper than equivalent-sized UK built boats. Although Ribeye seem to have relatively expensive list prices, my experience is that it is not difficult to get a sizeable discount.
Coming back to your original point, I went to the S'hampton Show this year to look at / test, even buy a RIB. However, as a lifelong 'rag and stick' man, I allowed myself and my wife to be drawn towards the sailing yachts. Came away with the feeling that whereas £20k would put me into a very nice 6.0m (ish) Avon or Ribeye, three times more would get me into modern French cruising yachts in the 32 - 34 ft range. It just seemed that the labour, materials and design effort to produce a cruising yacht is a lot more than 3 times the equivalent to produce a RIB (is this making any sense?) Anyway, I thoroughly agree with the point that new RIBS do seem somewhat expensive for what they are. Going for a used boat has to be the answer, as long as you accept that the number of variables in a RIB set up (hull form, engine type / power, colour, seating config. etc. etc. ) means that you will almost certainly have to make some compromises - I continue to comb the 'boats for sale' pages!