Are you connected to the internet?
If so, go do the Seasonal Events, and if you got a 200% login bonus you'll earn twice as much pr race (including A/B-spec races offline).
If you're not connected to internet, give your bob a X1 (if you have one, if not I could share one to you, but it requires internet connection) and do the Dream Car Championship, will earn you about 850 000 (double it with 200%) in 1 hour without doing anything.
Chassis maintenance does have effect yes, but not after every 300 km.
I regulary repair my cars every 5000 km or so.
About the Gallardo being expensive to maintain... Not really if you can do the seasonal events, as the seasonal events are way to overpaid.
And most sportscars/racecars costs about 500 000 to repair, Veyron is 600 000 if I remember correctly.
Hope that was answer enough, let me know if you need more info or anything.