Nice simple visuals, really like the silhouette of the time and player when finishing a round, but the balancing and time to get upgrades was a bit off, I would probably have it so you can finish the first wave after 2 upgrades and make it so that you get upgrades faster, even if that means the upgrades are cheaper, to make the player feel like they are earning more with every life instead of slowly building up their money to buy one upgrade. Would also make the game be completed faster so players get to see more enemy types and stuff faster.
But other than that really cool, simple incremental game!