It depends of what kind of services you require from a wedding planner. If you want to do most of the planning yourself but just want help with the On the Day then you would be better to have just an On the Day coordinator. I offer this service as well as Venue Decor & sourcing. It can take a lot of your bidget to hire a planner to do everything so it really depends on what you actually need a planner for.
You really want to determine what you feel you need a wedding planner for
A real wedding planner can look after you and hold your hand right the way from the engagement through to your wedding. They can organise hotels, transport, guest accommodation, cakes, cars, carriages, fireworks, catering, drinks, bars, venue dressing, staff, marquees, photographers, , hairdressers, makeup artists, videographers, flowers, tables, chairs, chair covers, generators etc.. On top of that they can asist and help deal with wedding lists, guest lists, invitations, stationary, table planning etc.
Good wedding planners will listen to you, and create the wedding of your dreams, and have an abundance of creative ideas for you. They will be incredibly well connected to suppliers that are known to be reliable and excellent
One of the most important things a good wedding planner does is manage your budget and work to a budget. They will negotiate for you, and usually save you a lot of money. They can also assist with the legal end of things, registrars, vicars etc. In other words they can deal with every aspect of organising and managing your wedding. One of the very last things a wedding planner does is manage the day, that management often begins a few days before the wedding as there are a lot of services to coordinate
My wife happens to be a wedding planner, and we work very closely together. From my point of view what I see is couples reducing their levels of stress and paying a lot less. As a supplier (wedding photographer) working at a wedding that has been professionally organised from start to finish is a dream, as I have couples that are just enjoying the day, rather worrying about doing things, and that lack of stress really makes a big difference to the couple, and all of the wedding professionals involved with the day - It just all runs like clockwork
To directly answer your question - wedding planners usually directly cost a few thousand pounds upwards, but generally save you a lot more than that! On the day coordinators charge from £250 upwards... you need to consider how much you are handing over to them, and how involved they are in the couple of days before the wedding. Some turn up on the day, some spends a week or so finalising things with suppliers