Well as a minimum your drone operator needs to have permission from the Civil Aviation Authority to carry out commercial work. This will mean that not only has your drone operator been trained in area such as Air Law, Understanding Airspace, Navigation and Weather Systems but also they must prove that they are insured to carry out commercial work and have an oppositions manual with processes for unforeseen situations, like a drone flying away. In short, getting a CAA authorised Drone operator show that they are committed to carrying out work safely and professionally.
How much should it cost? from £50 - £500 dependant on the amount of preparation required. If you want to fly a drone down a high Street then there is a lot more work needed beforehand than if you want a photo of your field from 100metres up. Prices can easily exceed £500 if you are looking for large detailed surveys or complex film work but for most jobs the 50 to 500 pound mark is a good guesstimate. The £50 end would normally depend on sharing the cost with someone else, for example if you and 3 neighbours wanted pictures of your house, or if the pilot was already working in your area you might get it cheap.
For a more detailed look at costs and what else goes into being a drone pilot have a look at this Drone Safe Register article.