I'll do "my" maths again for the benefit of anyone else.

Typical ET flight to say Germany is say £150 is booked reasonably far in advance (peak Friday there, peak Sunday back).

RFS is £35 plus 8k or 9k. So, rounding slightly, 8k save ~£120, thus 1.5p per Avios.

Needless to say, fares can be much higher than £150 and RFS' are still available. Even with the Gold Priority Reward (double avios, 30 days out, guarantees Avios seat), I've saved some serious £. (think my example was a MRS - LHR flight which they wanted €300 for o/w and I got it for 9k and £17.50 instead).

So even if not just topping up, this can be a good deal to get RFS' etc for those not sitting on a mountain of Avios.