not bad! i like the idea behind this game.
kinda reminds me of that old anti tower defense flash game i played years ago.
the current version has a few minor issues though.
first, your income can get into the negatives and if you spam click the high hp units too much and you can get stuck with no income, basically a gameover situation. i suggest setting a lower limit of 1 income per 5sec and a buy limitation for those units if the income would fall below that limit.
second, currently the only way to beat the game is spam-clicking those high hp units and earnestly, my hands hurt after clicking a few hundred times as fast as possible... so i suggest adding a wave send function where you send a certain number of units over a certain interval. maybe add upgrades to shorten said interval and add a formation/composition setting, along with stat upgrades.
third, general lack of content. for the formation setting to be useful, there need to be more unit types. i get that 48 hours is little time to flesh out the game, but now that its past the contest, please continue working on it!
fourth, is pretty much an extension of third. the visuals. i don't mind minimalistic graphics, but i still want at least SOMETHING to look at. just colored squares and circles just don't cut it.
with all that being said, you have already finished the hardest parts of game development, so keep up the good work!