Thanks for all the responses, so based on the (admittedly small and probably skewed) sample FT+ is on average a marginal cost of aprox. £1/month for current users.
(Ignoring things like the opportunity cost of cash and assuming ISA/SIPP would be paid regardless)
This might not be the right question, though. Especially if asking this question is a prelude to changing things up.
I only have Plus because it works out cheaper for me than paying the ISA because I have spare money to earn interest on. That said, the only plus feature I use is limit orders, and if the pricing structure for Plus changed drastically, I’d drop it and just go back to paying the ISA fee or look at other brokers and reassess if FT is still the best option for me.
Also, I found it hard to answer the actual question because your description of how to calculate the cost neglects the interest or earnings you could get from the cash if put elsewhere.
I consider freetrade plus to be virtually free and in some ways actually making money.
£4000 3% interest. That £4,000 would be in Marcus at 1.2 %. Therefore £72 from freetrade. My isa in Hargreaves Lansdowne works out at £3.75. that works out at a total cost/savings of £117. Hargreaves Lansdowne £11.75 per trade. How long before I can reinvest dividends at reasonable cost? Hargreaves Lansdowne 1% £1 minimum £10 maximum. I now have no money drag. The latter makes me very happy. Investing a spare £1 with no cost is brill.