Locksmith prices can vary wildly as with any other business or service you use. The time of day, opr day of the day of the week will affect the price too. A weekend charge will be greater than the weekday charge. Equally, calling someone out at 3AM is going to be much greater than a call within standard working hours.
Equally, holiday charges, bank holiday charges etc these are all times that the locksmith is sacrificing to help you gain access and rejoin the fun (or sleep if it's 3AM). It's only fair if your being kicked out of bed or sacrificing family time that the locksmith be recompensed.
However, the locksmiths will usually make it clear as early as possible before starting the job on what the costs may be. A rough estimate is always possible and if you call an experienced locksmith company they will be able to quote once you provide an image at a 90% accuracy rate (or at least we can). But please remember he (0r she) is working from the information YOU have given so be as precise as you possibly can be.
Stay safe you crazies!!!