My advice is to add up price up to year 5 or 6 (prep basically) adding some for annual fee increases.
Do the same for senior/secondary (year 7 -13).
Then you'll have a feel for the size of the investment. You can just think about the cost per month or per term which never sounds as bad.
Have a look at your local primary/ies. You might be pleasantly surprised and it's fine to move if it doesnt work out. It's probably less scary for you to move that direction too if you haven't been to state yourself. Lots move into y6 of the independent locally before moving up to y7.
Smaller isn't always better. Friendship dynamics can be challenging in older prep and a bigger school can help.
The extras can sting a bit. Not really because they are unaffordable compared to the fees but because they further eat into the fun money and the buffer money that makes life less stressful.
I remember looking at a bill with extras for trips, music lessons, meals and £10 for a compulsory homework diary. Of course we could afford the diary but but but...
We've used every type of school as a family. One of my DC would say the very best teacher was head of English at the comp, only narrowly beating a truly inspiring History teacher at the independent. Art teachers amazing at both. Geography really good at independent as more freedom in early secondary. Worst teacher was at the independent and wins that title by falling asleep often.
Best things about the independent were the choral music (difficult to replicate in state unless you live by a cathedral without attached school), the quality of the food and the amount of exercise (not sure DC would agree with the exercise bit).
Worst thing about prep according to my friends is the amount of prep (homework). You're paying the fees and then having to supervise or support homework in precious family time, more than most state primaries.
The bright, talented kids are every bit as bright and talented in state as in independent. Every bit as motivated, as nice and as well supported by parents. Unless your state options are truly, truly diabolical but I suspect not.
PP made a valid point about planning in a bit of flex in case of future siblings or step siblings. You could save the secondary fees while she is at state primary and then you'd have security knowing whatever happens work and health wise, the fees are covered. Some schools offer a bigger discount for more years in advance. Then if something went wrong you wouldnt be desperately hoping your child was seen as adding value to school life (arts, academics, sports) to merit a significant discount.
Last thing, in my experience, you need to direct queries about fees, discounts and scholarships to admissions and the bursar. The head may have some influence but the schools are businesses and rightly have to balance the books.
Sorry that's very long! Good luck.