you’d have to dynamically calculate the rental total for each row based on the date filters and calculate it on the page rather than in the backend (assuming user is changing the filters often)

you’d be better to do it based on a daily rate also since months vary in days so a month with 31 days would be less per day than a month with 30 days (or 28)

to achieve this per row I’d add a few calculation groups on the page/row

days = end - start format as days (page level)
rental total = days x rental rate daily (row level) format as currency

now when user changes date filters it recalcs the days which then recalcs the rental total for each row

you may also need to calc days per row and use max/min date to compare filter date to the rental records date and use the appropriate date for start/end (ie if filter start > rental start then user rental start date)