So from my current playthrough which is sitting at around a full day of playtime it seems as though the game hits a cap when you buy the second to last particle. At this point the upgrades are too minimal and are not worth the effort so I simply wait until I finally scrounge up enough for a small upgrade.
All I know is that the progression curve of this is pretty insane... For the fact that I spent about 20 minutes on my first run and after that each run cuts in half... I think there needs to be a bit more progression :D Because after the first hour of playing this it's less than a minute and a half to get to 6k+ depth
 "Idle accelerator"){kind=icon}
 "Lasers & Dirt 2 - BETA"){kind=icon}
More content is always in the works. But the layers will be the last thing I'll add! :P