That is like asking how much do restaurants charge for dinner. You can get a burger from the dollar menu at some of the fast food places for - well, a dollar. Then you can find a burger on the menu in some high end restaurants for over $20. Then there are other things on different menus all together. Probably more important is that the person who frequents the dollar menu is highly unlikely to be found in a high end restaurant or vice-versa. It is no different in landscape design.
The first thing that will limit how much you can charge is the level of service you are providing. You were probably told that it is common to get 10% of the price of the overall landscape as a designer. What you probably were not told is that includes contract administration which is being completely responsible for finding contractors, inspecting and approving their work, being fully responsible for the completion of the job and everything that comes up during and after it. Most landscape designers are not hired to do all that, nor are most capable of doing all of that.
Most designers do only a layout plan and hand it off to the client to find their own contractors and manage the job themselves. That is a plan drawn to scale showing the location of plants, hardscapes, and other features on a single sheet of paper. The reason that this is the most common practice is that this is what most clients are wiling to pay for. You can't make a living trying to do something that people are not buying, so more designers offer this level of service than not.
Other levels of service include construction documents. These are plan sets and specifications that cover the most minute details of how things are to be put together and exact materials to be used. This is a huge amount of work in many cases and the designer takes on larger responsibilities if the plans are followed and something fails. Obviously this work takes a lot more time ($) and exposes the designer to more risk ($). That drives the cost up as well as protects the consumer by having a more guaranteed outcome. The problem for the designer is that he needs a much deeper knowledge, has to sell a more expensive service, and has to protect himself with costly errors and omissions insurance.
The third thing that is going to limit your price is your market. Your market is whom you are selling your plans to rather than just the geographic area you are selling in. The most realistic definition of "your market" is the people on the other end of the phone line when your phone rings. You can't decide what your market is. You have to position yourself to be worthy of a particular market. You can be fully capable of designing high end residential landscapes, but if those clients are looking somewhere else, or don't have great confidence in you if they do find you, or find others to be more experienced then they are not "your market".
"Your market" is going to have some opportunities and some constraints. The smaller the budget, the less likely they will pay for a landscape design. It takes a certain amount of time to do even the most limited plan. Bare minimum is two hours if you include driving to a site, meeting the client, and scribbling on a napkin. How cheap can you charge for two hours? That has to be at least $100 no matter how inexperienced. Someone with a budget of $1,000 is not going to drop more than that for a plan and probably won't spend a dime. Someone spending $10,000 on a landscape is not going to spend more than $200-$300 on a plan and also is unlikely to spend a dime on it.
You really need to know how much you need to charge to do the level of service you are going to provide. Then you have to ask yourself if "your market" is going to pay that much.
I have a certain way that I work which requires detailed measuring and accurate drafting. This takes considerable time ($). I produce 24"x36" black & white layout plans, usually at 1"=10'. I have found that the minimum that I can charge for any plan is $900. If the job does not warrant that much for design, I simply don't do it. That cuts off a huge amount of people who want to have a landscape. They are out of my market. I find that the total budget cutoff for people willing to pay for design has a floor of about $20,000. If people are not investing more than that, they are not going to invest in design.
I usually charge $1,500 for a residential landscape layout plan, but range between $900-$3,000. Many of these include multiple level retaining walls, swimming pools, patios, walks, driveways, fencing, pergolas, .... The built work ranges from $20k - $200k.
You have to remember that I have been doing this professionally for 30 years, have a degree in landscape architecture, worked in a civil engineering office for ten years, have an extensive resume, am a licensed landscape architect, and can talk a dog off of a meat wagon. I also make my living with a full time in a civil engineering office, so I have the luxury of not taking on landscape design work that don't pay enough or that I don't want to do. Those prices might sound good, but I only do about a dozen landscape plans a year (could do more if I did not work full time, but don't think I could replace my job income and benefits).
It is a very competitive profession. You have licensed landscape architects, people with landscape architecture degrees, horticulture degrees, landscape design certificates, home gardeners looking to make extra money or just apply their hobby somewhere else, lots of people losing corporate jobs with graphic skills who think they "can do this", and the biggest competitor of all is the very well experienced landscape contractor who knows what he is doing - is willing to do the design cheap in order to land the construction job and is more likely to get the phone call than either you or me.
Don't expect to get rich quick.
Your best use of the skills that you are learning is to use it to sell install services. When I was in design/build, I found that once I did the design, the install was mine to lose. Selling the design can almost translate to selling the build. Don't price yourself out of making the real money on the build. However, don't design for free. That does not work. Anyone will take something for free whether they value it or not. You need them to show some commitment. If they give you $100, they probably are not taking three free designs from others. It lets you know that they are focused on you.