I found these 2 articles interesting.
Easier to read chart (prices in pounds): https://www.expatica.com/education/lang ... rn-728953/
Same language list (prices in USD): https://www.dynamiclanguage.com/cheapes ... -to-learn/
Both sites take the number of hours needed to become fluent and the average cost of tutoring (per hour). Although you can become fluent without a tutor, the prices do give a good estimate of how much you would need to spend on other methods (textbooks, apps/websites, immersion, online classes, books, audiobooks, etc.).