Its cheaper to save your money and buy all the upgrades at once although it would take a whole lot longer to get anywhere. Needs to subtract the price of the last upgrade from the next one after you buy the previous upgrade.
Wasn't a bad game until my 5th time hitting the Geek, the game glitched and sent me on a unending spiral of the Geek going to the clouds and smashing into the ground. Speed keeps rising and don't see a end to this, only thing I have bought out of the store was the first Weapon and the game became to easy.