Well, have they defined the scope of the work, or is that up to you?
It should not be a mystery if you know what the work entails. If you don't know what the work entails you should define it and price out what you have defined for the scope of work.

What about it leaves you feeling like you don't know what to charge?

I usually charge a flat fee for my residential designs...commercial is sometimes more difficult to estimate. Are you and your clients comfortable with quoting an hourly rate and "guesstimate" of the time it may take for a concept? That way you have a bit of a "fudge" factor if it takes more or less time than you thought.

Who would charge an hourly rate for a design project? Can't imagine that.

Just make sure that you know FOR CERTAIN what your fee covers. Some clients think that they will be entitled to twenty complete changes, lol! (Commercial clients can be especially difficult, sometimes.)

"make sure that you know FOR CERTAIN what your fee covers"

The best way to do that is to write the contract yourself. This is one of those things that separates someone who can do good design work from someone who is a professional designer. Designing is only a part of the business.

On the Design Forum it has been asked several times "what do I get from a professional designer?". The amazing thing is that a lot of people respond with a list of what they think is some standard things that are to be received from each and every landscape designer on each and every landscape design.

It is a good practice to prewrite several hypothetic contracts and reread and rewrite them looking for holes in them and look for things that make a client feel uncertain. The best contract is one where both parties can clearly see the limitations and responsibilities of the other.

I like to define the job in terms of specific areas on the property, a generalization of what the design will include, how many meetings, how many revisions, what sheets will be included in the set of plans, and hourly rates for things that are not covered in the contract.

If you don't do these things in a contract and you include a patio, are you then required to draw up a cross section detail and specifications for that? Maybe you don't think so, but maybe your client does. You spend three weeks designing a landscape and then the home owner sees something on HGTV and has to have it, but you have to redesign half the project to make it work. How do you cover it in a contract? You have to think about these things.

Who owns the plans when complete? How many sets of plans do they get? What happens when the architect changes the building half way through your design because the planning board did not accept the original? .... Do you have to go to the regulatory boards and present your plans?

If the other party has a contract for you to sign, you have to watch out that things are not too general. "Meet with the general contractor to discuss revisions" might make you think that means one time, but it does not. You might go to ten meetings. Revise the plan - how many times and to what level of satisfaction?

It is not easy, but designing a contract is like designing a landscape. You assess the situation, the people's needs, what you can do to make it sell, what you can do to make it work well for you, and put it all together.