Health and Main Cannon stats can be upgraded from the start of the game at a low star cost (the price rises after each stat upgrade), but the rest of the upgrade options are quite pricey at the start of the game. Instead of saving for those, upgrade Health and Main Cannon a fair amount to make your in-game experience easier. Making the Precious Cargo in-app purchase helps you put a large dent in acquiring the first two upgrades, allowing you to spend your time on the ones that are harder to get.