My first instinct would be to determine locally what landscape companies sell labor for. Ask as if you were a customer, checking around, rates or something along that line. I would base my number on that, competitively and discounted where you can, see fit, relatives, friends, again, only if you can.
You are smart to own and use your own equipment, I see the same exact thing, much better to have what you need for productive use.
My philosophy has always been, to charge a fair price for a job done as well as you are capable, being as competitive as you can, but not gouge.
With any of this you will have some overhead, operating costs, incidentals, sundries etc., frankly, $5/hour was fine in 1983 for labor, personally, I'd rather sit idle than use my equipment, tools, etc. to perform labor intense work that does not provide enough cash flow to cover overhead and fair labor, realizing an honest profit at the end of the day. It may sound arrogant but truthfully, you become a charitable volunteer. Nothing wrong with cutting someone a break, doing a small job here and there, no charge, always a nice gesture and reflects well on your character, you still need to put food on the table and pay your bills, volunteer work may get you a meal sometimes, it won't pay the bills, I don't get the logic. Say you were retired, have enough income, then do low cost, more charitable work, to keep active, that would not be so unfair.
I've done a fair amount of this while in a similar position, more tools and equipment involved, as well as a larger scope of work, and have broke even, on hard labor intense jobs, at the end of the day, it does not add up, and is why I do not do much of it anymore. The profitability is minimal, yet the labor intense. It takes much more out of you in the mid 40's than it did in the mid 20's, and you have to use care with safety/injury and similar risks.
Some people are quite ridiculous, I had a customer who wanted us to do some things totally against the local building code, and one of the inspectors lived across the street ! That job material was delivered and the work started. Did a 180 degree turn, no way, you own the material now, pay up and we are out. The profit was marginal, it was a long time friends parents friends so they got a break. They had to find another contractor, it was not worth our reputation with the municipality, which was impeccable. They clearly were not happy unless we defied the code by what they wanted done, once it was determined the structure we were intending to repair, really needed to be demolished and a new one built, which was stated right up front as a distinct possibility. They knew.
More complicated and apples to oranges, but I think its unfair to do work at a deficit no matter what it is, whats fair is fair, can't be tilted one way or the other, or whats the point of getting involved ?