When I asked myself this question years ago, I landed on a through that can help anyone answer it: "Are my clients paying me for my tools, or are they paying me for my skills, experience, and expertise?".
In many cases, clients do not care what tools you use, as long as you get the job done and produce the results for which they hired you. As someone has stated above, "Someone using free tools but has the skills can outperform one with the best paid tools but poor skills" If the answer to the earlier question is the second option, I do not see any issue with charging clients when I use free tools.