Modify your estimate for:
- Multi-day trips
- Trips to places with airports nearby
- Times there more people travelling than will fit on your Cessna
- Times multiple people are flying to different places
Are there parking fees at the airport?
How about maintenance costs?
I don't doubt that a lot of people could fly GA cheaper than commercial, but I don't think owning your own company plane is likely to work for small companies. And in any case the calculation is a bit more complex than you presented.