(edit) for those that are friends of chance wait until the 22nd of any month for the daily challenge and fully upgrade the scourge. the rest of the game is potato.
here is what ive learned on saving money in this game and how to get rich fast:
1. If you are pro enough, you only have to upgrade the A7 engine once to get past the first levels.
2. After comlpeting Pelham one two three upgrade the miester to full capacity(with the leftover money get some new weapons).
3.Pro's only! complete levels until you get the giant(if you can do it, wait till you get the police car since its much better).
4.This is the hardest...complete barrage dam for the Panini car and switch to that.(again leftover $$$ goes to better weapons)
5.survive until you win the game!
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 "Infectonator : Survivors Alpha DEMO"){kind=icon}
 "Burnin' Rubber 5"){kind=icon}