I recently reached Champion level (a few days ago) so I still have a good idea of how to jump up there. Through normal play of the game, I got myself to about 300g, but I had to "boost" myself up to 450. There are quite a few "easy" achievements to get, it may just take some time. The first thing to do is go to a house, then go Info>Achievements and eventually, "LB" and "RB" to scroll through specific achievements to know what you do and don't have.
There are a few achievements that are earned by collecting 3 of each of multiple car companies. To do this, I sold my McLaren F1 GTR and used that $750K to cushion the money absorbed by buying/selling nine or so cars. A list achievements you may not have...
(30 points) SINGLE PLAYER VIC. - LEVEL 4 - Win the Gold cup in 40 Single Player challenges.
(40 points) SINGLE PLAYER VIC. - LEVEL 5 - Win the Gold cup in 60 Single Player challenges.
(15 points) SUCCESSFUL MISSIONS - LEVEL 1 - Successfully complete 3 missions (Top-Model, Hitchhiker, Vehicle Transport or Courier).
(25 points) SUCCESSFUL MISSIONS - LEVEL 2 - Successfully complete 15 missions (Top-Model, Hitchhiker, Vehicle Transport, or Courier).
(10 points) INSTANT CHALLENGE VICTORIES - Take 5 victories in Instant Challenges. As you are driving around and come across other live opponents press ‘A’ to flash your lights and enter the instant challenge mode.
(20 points) FERRARI COLLECTOR - Own at least 3 different FERRARI.
(20 points) LAMBORGHINI COLLECTOR - Own at least 3 different LAMBORGHINI.
(15 points) ASTON MARTIN COLLECTOR - Own at least 3 different ASTON MARTIN.
(10 points) CHEVROLET COLLECTOR - Own at least 3 different CHEVROLET.
(10 points) FORD COLLECTOR - Own at least 3 different FORD.
(10 points) BIKE COLLECTOR - Own at least 3 different bikes.
(10 points) BEN SHERMAN COLLECTOR - Own at least 10 Ben Sherman items.
(10 points) *ecko COLLECTOR - Own at least 10 *ecko items.
(35 points) SEASONED TRAVELLER - Cover 1,000 miles (1,600 km).
(20 points) OWNER - Own at least 2 houses.
(30 points) CONFIRMED COLLECTOR - Own at least 10 different cars.
(30 points) MOST VARIED COLLECTOR - Own at least 20 different cars.
While you may have some of the above achievements, they add up to 340 points, so try to get what you can from above...I found that they're the easiest to get.