Fat WalletG 5

Fat Wallet - Collect over 1.000.000 Coins (5 GamerScore)

After finishing the last of the Series, you should be well after the half-mark to the 1 million requirement, in the range of 600-700k.

As only Series events give you respectable amounts of Coins compared to the Stunt Parks (Quick Race events do not give you any Coins, save for 500 Coins for any and each Car Part-replacement), I'd recommend replaying the 300 HP Series as these give you the largest payouts (first three events give you around 60k, while the final one pays out around 130k) with a negligible increase of difficulty.