Boosts are expensive and may not be worth the cost. The Fart Saver is recommended because it allows you to continue after dying. Double Gems is only worth it if you have an excess number of gems to collect. You need to collect 125 per run when you have it activated to break even. Double Diamonds is also worth purchasing when you have the gems available, however it is risky and will on average only net you a maximum of 3 Diamonds.