Rush has two kinds of coins, and they work differently.
Gold Coins are for fun. You get some free when you sign up and through regular bonuses, and you can buy more if you want. They have no cash value and can never be redeemed for prizes.
Sweeps Coins are free sweepstakes entries, and you can never buy them. You can get them through the Daily Bonus, by mail, through promotions, or as a free extra with some Gold Coin packages.
Sweeps Coins you win through play can be redeemed for prizes, and each eligible Sweeps Coin is worth a $1 prize. Buying Gold Coins never improves your chances of winning.