It hasn't always been £15.50, although it probably was last time you booked. It used to be £17.50 + 4500 avios for the last few years, but when they introduced the 50p + 8000 avios option in the last year or so the 4500 avios option price fell by £2 to £15.50.
When I said "always" what I really meant was that the price had been £15.50 + 4500 Avios for the last 20 or 30 RFS tickets I booked. I remember it being £17.50 before that. And now I realise I have absolutely no idea what is going on with the price of RFS tickets.
(And isn't a "cash discount" a reduction you get for paying with cash? Do they just mean "saving"?)