Quite a satisfying game to play. The way the game is currently structured, especially with how certain upgrades scale, the basic damage, spawn/fire rate, and damage reduction upgrades seem to be the best upgrades for quickest progression, which is a shame because the other upgrades seem like they'd lead to interesting dynamics. Achievements, challenges, alternative/iterated methods of progress, etc., could easily fix that. Regardless, the overall zen effect of having dozens of white orbs drifting about the screen cannot be overstated.
Edit: Playing with modifiers after winning the game, and wow, are they fun! Maybe a little notice could be added that lets players know the modifiers and price scaling isn't permanent between switch-ups, because I initially thought I'd completely ruin my progression if I started messing with them too soon.