the best way IMHO to price anything is getting a good estimate on how long a project will take you, a day? half a day? 4 days? Doesn't take too much skill to trim but i think it should be priced more than what you make per hour mowing.
Most people can trim, yes there's a proper way and time to do it i.e safer for plants not in summer but that's when most want it trimmed from Spring growth doesn't look crisp, pruning things to the collar so it heals over rather than leaving a stub that'll rot and let rot into the tree, maintaining a central leader for pyramidal shaped trees, removing suckers, something like virginia creeper taking over the plant should be traced down to the root and uproot as much as possible, keeping clean/sanitized blades for certain plants to not introduce disease or something from other plants, things like that can be done wrong though.
I actually kinda like trimming, it's the clean up that can be a pita but sometimes you can mulch mow and blow small clippings somewhere beneficial like right back under the tree as natural mulch, thorns can be a pita to clean.
But basically anything should be priced higher than a mowing rate, most can do the trimming but they just don't want to or make the time to or buy and maintain the tools for it, same with mowing. I mean, some are making 800+ a day just mowing, but also your area can dictate a lot - if it's an expensive area like nj/ny/california people make more money there, so your price should be higher than somewhere like in the middle of no where middle of the country where you can get a brand new 2 story house for like 250k that would cost 650k in an expensive area. So find your state/area average income and go from there, most aren't running a business for average income there's lots of easier jobs that pay average, in an expensive state/area some price for a 6 figure-rate, some around 80k-rate, many charge something like a 150k+ rate.