In fairness to the solicitors, it can depend on the complexity of the probate, the number of beneficiaries, executors etc that have to be dealt with. Done it a couple of times, one was very straight forward, 1 executor, 2 beneficiaries and a straightforward estate. Other one was a mare, 26 beneficiaries, (some of whom died during the process), a farm, no tax paid for years, bank accounts all over the place, money litterally under the mattress and it took over 2 years to get everything sorted and cost a small fortune but the solicitor had to engage an accountant and tax advisor to resolve.
So no certain answer, will depend on circumstances but they should provide you with a proposed bill of costs before they start