Now this question is one that I see couples asking more than any other when it comes to planners and honestly, it really can be hard to gauge whether or not someone is overcharging you. There's such a wide spectrum on which every vendors prices fall and you could talk to 4 different planners and get quoted $1,200, $4,000, and $10,000 for the same event. If you want the short answer as to why, its that usually you get what you pay for at the end of the day. Notice the emphasis on 'usually' though because there are definitely those out there that severely undercharge for amazing service and those that will overcharge you for the bare minimum. There are so many variables that go into a planners pricing but even still, quantity does not always equal quality.
The best advice I can give you about HOW to make sure you're actually getting your money's worth is to ask the right questions in the initial consultation!
A good planner will know what makes them special and will be more than happy to explain to you why if they haven't already. Ask them this and pay attention to if they have a unique answer or give a very general response.
Ask them for examples of what they're offering and I mean more beyond just pictures of a wonderful event. Have them show you something like a previous timeline or have them really go into detail on how the planning process will run with them.
Don't leave without asking for a copy of the services and quote they proposed so that you can look at it side by side with other quotes. This will be the easiest way to make an informed decision without forgetting the little details of each offer.
Just don't be afraid to ask. I always tell couples that information makes decisions and not time. Don't leave the consultation unsure of whether or not your being overcharged because you won't be able to truly know without the proper information from the planner. There is really no right or wrong answer to how much a wedding planner should cost, but if you feel like you're being overcharged or you're unsure why one vendors prices seem higher than others, all you need to do is ask about it. There is usually a really good justification behind every planners investment costs.