That really depends on who you hire...solicitors fees vary with their expertise and seniority and the kind of case, ditto barristers fees, expert opinions, expert witnesses etcet. Of course if you win hands down you can ask for the other side to pay your costs if its a private case (not criminal) but the other side can do the same. Start with "lots" and go on from there. A couple of years ago we were involved in something where the value was around £150,000. We spent about 8,000 on solicitors fees and letters and specialist opinion and were advised that the case could go either way and if we lost and they were awarded costs (which would be likely if we lost and vice versa) it could cost us more than an additional 20,000 or so as they had employed a London solicitor and their barrister was likely to be an expensive one.
I hope this helps a bit.