Some of the fuel prices are astronomical - especially the HERC box keys. I would be very surprised if the majority of players have bothered or been able to open more than one box, for which a key was given to us.
Maybe fuel gets more common on missions past a certain level, or something, but I've found hardly any so far.
waves are way too big, just makes it boring. the reason gemcraft is so good is because the waves are a challenge because of the different types, rather than just making them all huge groups.
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