That is just a mortgage amortization formula. At 5% interest per annum, paying $114 million a year, or about $9.5 million per month, $6.5 billion can be paid back in 27 years. The bridge pencils - and pays for itself. This assumes that the $114 million represents net revenues.
It's not. Your calculation ignores salaries and maintenance on an 8+ mile long heavily used toll bridge. That revenue number is even with cars paying $6 during peak on weekdays and $5 on weekends.
If Texas wasn't so anti-public services, it would actually be the best place in terms of major US cities and flat cheap empty land to try high speed rail in the US. Dallas-Waco-Austin-San Antonio-Corpus Christi and El Paso-Midland-Austin-Houston would connect 90% of the state at the capital with clear interstate expansions on all sides (Tucson, OKC, NO)
Let's see... American Airlines headquarters... Fort Worth. Southwest Airlines headquarters... Dallas. Continental Airlines headquarters... Houston (recently moved to Chicago after merger). Rail ain't gonna happen in Texas.