No need to guess.
You can check mygranturismo.net for your own figures after you input your garage.
Mine stands at 490 million excluding multiples of some cars and tickets (my garage has 1215 cars and 1085 models with another 50 unused tickets). Still short of X-1 prototype, DLC Camaro Edge Special, FT-86 Concept II and XJ13 Jag Chrome line. Except for X-1 Proto, the rest are not available in my region (Asia).
In short, the total figure should be somewhere around 540million if you live in Europe (as such XJ13 chrome line is available) bought signature edition of GT5, every DLC and got the FT-86 concept II instead of Kart SPL and got the pre order Camaro and won X-1 proto via Vettel Gold.