This might have been a fair price, but sadly there are a number of rogue companies, often advertising themselves as "24 hour locksmiths" and charging whatever people with a sudden emergency will pay. It's said that there are people doing the same as emergency plumbers.
Often they are nor real locksmiths and will spend 59 minutes pointlessly tinkering with your old lock before announcing that it needs to be replaced, so they can charge for two hours work plus parts.
If the door is wooden, any carpenter can fit a replacement lock, preferably a British Standard one; and there are people who specialise in plastic door and window repairs who can do the same.
There is a trade body called The MLA
Depending on the size of your town, there is usually a retail locksmith business that sells locks, safes and sometimes burglar alarms, and usually offers a fitting service (sometimes after the shop is shut) or will know local tradesmen. They are less common now that DIY sheds sell locks cheaply.
OOI, if you have screwdrivers and are fairly handy, it is fairly easy to take out an old lock and replace it with new. The trick is to get one of exactly the same size, so preferably the same make and model. Otherwise, chiselling may be required.