Presumably a tradesman has knowledge and experience with these blockers, which for them should be largely "known unknowns" and easy to fix at that since they have all the right equipment.
So why do they still charge so much? It's not just "knowing to drill the hole in the drain" but overhead like equipment (they need to buy all the expensive pro stuff, and keep it in working order; whereas you might just need the one tool to get your job done, they need all the tools to get all the jobs done), vehicles, gas, etc. And of course the down time traveling between jobs, office space if they have an office, employees, etc.
All these things bring their prices back up to much higher than it would cost you to do it yourself--if only you knew what to do and had the right equipment.
So why do they still charge so much? It's not just "knowing to drill the hole in the drain" but overhead like equipment (they need to buy all the expensive pro stuff, and keep it in working order; whereas you might just need the one tool to get your job done, they need all the tools to get all the jobs done), vehicles, gas, etc. And of course the down time traveling between jobs, office space if they have an office, employees, etc.
All these things bring their prices back up to much higher than it would cost you to do it yourself--if only you knew what to do and had the right equipment.