I've found an easy way to figure out when to buy what is to peg the price of something to how much it produces. So the unit that produces 8 g will cost 8 billion, the unit that produces 12 g will cost 12 billion. It's good for when they don't evenly double.
I'm noticing something that tends to happen is mainly the front ships get destroyed, and they end up being the only ship from the harder wave, which makes completing the wave easier.
I like being able to control the guys because it lets you limit how much weight is on a bridge at once, but I think it would be nice to have an option to invert which side of the guys you click on. To me it feels like it would be more natural to click on the guy's 'back' and have that push him forward, maybe it's just me though.
I really hope you don't get discouraged by the negative comments and make a level pack or a sequel that's longer than this. Maybe with some additional obstacles or features, like actually using your sword or stealth would be interesting, and this is really begging for user made levels.
barrels are a better weapon than the rocket launcher >:| once theres fire near you you can just keep dropping barrels for an endless chain of explosions