It's 5,925,000 rep to get to level 50.
A fast method for earning rep (and cash) is to play short hard difficulty races over and over. They give the same amount of rep no matter how many times you complete them. It's 19,500 rep plus whatever you earn from performing actions like drifting.
Fueled By the Fire is an pretty easy time trial with mostly straightaways and only takes around 90 seconds to complete. Half Past Ignite is under two minutes as well if you're decent at drifting. You'll get around 30k total each time.
After you've completed the event, you can teleport back to the event and repeat as many times as you need to.
I was around 43 when I was done the story missions. Then it was a couple of hours of Fueled By the Fire to get to 50.