It's not really true that RF seats are hugely more expensive than FF seats if you're comparing like with like.
For example the cheapest FF seats start at about £60, but Halfords also have a RF seat up to 105cm for £60 which quite often goes on offer for £54. I don't know how tall the shell on that is so can't say if it would really last to 105cm (around 4) but it might do.
You can get other basic RF seats up to 105cm by Joie or Graco for about £100-150. This is around a similar price to a basic or generic FF "123 type" seat (harness up to 105cm, then high back booster seat) which is fitted with isofix.
Also the many generic "all stages" models you find are around this price (and often advertise RF up to either 13kg or 105cm, though I am not keen on these.)
Then what I would think of as a good quality FF seat up to 105cm, (with or without booster conversion) costs about £200, sometimes on offer for £170ish, sometimes up to £250 depending on exact model.
Most of the popular good quality rotating seats are (similarly) around the £200-300 mark and these can do rear facing or forward facing.
There is a discrepancy once you get into the old what used to be "Group 2" weight band (15-25kg/100-125cm/~3-7 years) because there are barely any FF seats in this group and the one which is still available is in the "Generic FF seat" category so is about £100-150 depending on offers. (Joie Bold is no longer available).
There aren't any ERF seats up to 125cm which are comparable to this. There is just no market for it - you can't mass produce them in the same way, and supporting the size of a larger/older child is not as simple as moving up the shoulder straps, you have to secure the whole shell of the seat from flying around.
So then you have good quality ERF seats up to 125cm - which again are about £200-250. There are no FF seats in this category - the old Joie Bold arguably was, but is no longer sold.
Lastly you come into what I tend to think of as the high end luxury/niche category. £400+ (up to about £700 for the most expensive) and higher weight/height capacity.
On the RF side you have the Axkids, the Besafe, the Avionaut Sky and so on.
Forward facing - Cybex Anoris with the airbag.
Besafe have one which rotates up to this weight and can do RF or FF but costs about £700 once you factor in the base, so definitely high end there.
Essentially, it doesn't make sense to compare a £400 luxury Axkid ERF seat with a £100 FF seat because you're not looking at the same category. At least compare the £100 FF seat to a £100 Graco RF seat.