Think of it in terms of the difference between an architect and a building contractor. If you know you want a building constructed on your property, but you're not sure exactly what dimensions, or what materials and construction processes would best suit your site, you retain, and pay, an architect. Knowing the details that you don't know, and educating you to the point that you can make an informed decision about how to best develop your site to suit your goals is in the architects wheelhouse. Once you've been brought up to speed to the point that you've made that informed decision and have paid the architect to produce a set of plans, you can then call various qualified builders and show them those plans and ask them for an estimate of the cost to make those plans a reality. Since the specifications are all there in the plans, they should be able to give you a number, and won't need to charge you a fee to do it.
If the prospective customer has already made an informed decision about what to do with their trees, and has detailed specifications which encompass that work, then all they need is a price and you can give them an estimate...for free.
If on the other hand, they'd like to know how to manage their trees, or how to best solve a conflict between themselves and their trees, then they need professional advice. That's a paid consulting call.
It's the difference between being asked to write a cook book for a client....or being shown a particular recipe and being asked what you'd charge to bring the necessary ingredients and prepare the dish.
