Bill sent me down a rabbit hole, just because I am Curious George

http://nsa.thevoicesoftware.com:3000/survey_nonmember.html

https://proconnect.intuit.com/taxprocenter/practice-management/how-do-your-tax-preparation-fees-stac...

https://www.investopedia.com/articles/taxes/021717/what-will-i-pay-tax-preparation-fees.asp

https://www.pnwtax.com/1040-individual-tax-return-pricing

In  other words - If you aren't loosing clients because of the fee you are charging - You are not charging enough. 

Also keep in mind what home prepared software cost an individual to screw it up themselves?