How could I prototype a board game with a variety of units using abstract icons?
Rather than using story cubes (which as noted are probably going to run you a fair chunk in bulk and aren't necessarily customizable), you might be better off just picking up some blank wooden cubes such as the ones found here or here.



Probably a lot cheaper, and you can put whatever you'd like on them instead of having to settle for what's on the story cubes. You could also have some stickers printed, for example at Game Crafter you can upload designs and have them printed onto half-inch stickers: see here. 

Note that I personally have not used Game Crafter's stickers so I can't vouch for the quality, but I've gotten the cubes from Amazon before and they are pretty nice.