Well when I arrived at Los Angeles I was just 5,000 Bucks short of buying an SLS but after I earned that up (which didn't take very long at all) I saw an SL-63 AMG at the dealer which was at half the SLS's price so I went for that :P
Without any conscious effort of grinding, I've raised above 500,000 Bucks from around 180,000 when I bought my last car (SL-63 AMG) and the most I can afford comfortably so far is a Pagani Huayrayryrayrayrayryaryaryayr Huayra while still being around 200,000 Bucks short of buying the most expensive car in the game so far which is the Agera R.
If you just stick to one or two cars throughout, unlike me who's bought a new car for every Spec, then you should find no trouble at all in raising a lot of money. I do have maxed out my perks that bump up money and xp rewards from skills, missions and the like and I've played several PVP matches in which you get a lot (around 15,000) regardless if you even finish the race or not. Also there are hundreds of landmarks which reward you 2,000 Bucks each.