Some solicitors charge a percentage, some structure fees differently. It is worth getting a number of quotes to ensure value for money, once you remember that value for money means getting a competent job done for a fair fee, and not just getting the cheapest fee.
However, your logic is flawed. If you sell your house, the value of your assets has not changed, it is now in cash, not property, so if you die, the value of the estate is the same.
Financial planning for what to do with your assets when you die is a good idea, but basing it solely on getting a cheaper solicitor fee seems like a false economy