Threads like this are largely pointless because each school varies so much. You'd do better off naming the schools you are considering so that people with DC there can advise you.
There is a conception that private schools have loads of expensive extras, but this is not my experience at all. My DS are at a private school in South London, and the extras are hardly anything.
The uniform is not expensive - the blazer is black polyester, cost around £30. Any black trousers are fine. OK, the sports kit is reasonably pricey, but the PTA holds regular second hand sales, and there are only 2 kits to buy that cover all sports played. (1 games kit & 1 PE). I don't think it costs any more than the kit at the local grammars.
You can take packed lunches in, it's not compulsory to buy school lunches.
Music/drama lessons are completely optional - cost £280 a term. All other extra curricular clubs are free.
There are expensive trips abroad that you can chose to go on, but these are optional. DS1 is in lower 6th and has never gone on one (his choice) and doesn't feel like he's missed out by not going. There is a field day once a term that everyone takes part in, but I've never had to pay for those trips, or for any GSCE fieldwork day trips.