Either way it's a "how long is a piece of string" question. No-one can give you a definitive answer with a set figure I'm afraid.
There are things you can do to keep costs down, such as only using the sols when necessary and doing most of the work yourself. You can get lots of help from this forum and Wikivorce had a divorce consultant service to help you. I would also recommend a book called DIY Divorce and Seperation, written by a team of barristers and is available on Amazon.
It's worth focusing on mediation and putting your energies into that. It will take a few sessions before you get an agreement but it's far better than court both in terms of costs and sanity.
The initial court fee is £255. mediation was definitely a non starter for me, so I paid the fee and went to court.
Additional costs are racked up by an awkward ex.
A barrister is expensive.
I was given figures at my first free solicitors appointment of simple circa 2K, slightly difficult 5K and full blown all the way to final hearing 15-20K.
You can do a lot yourself....research, print forms off etc.
I had no choice because ex view was I should get nothing after 20 years.
As this was the case I thought I had nothing to lose then!