500M Defense puts it down to 2,
700M sends it down to 1,
Takes 1.5B to bring it all the way to 0. 
Lv 2 Billion
The reason why it speeds up is because all the other units finally dies, with the skeleton attacking every time, progression through the stage finally becomes a so much faster. The bug is good for spawning all units, but not so much for farming levels or money.
You know, I've been waiting days since you left that series of comments about the "Bug" hoping you'd either elaborate with more details on how to recreate it or someone would get curious and ask you about it, since neither happened I created an account specifically to comment and ask you, shot in the dark as I don't know how itchio works with replies and notification, or whether or not that bug was repeatable somehow, it's a nice find, I've beat the game myself a while back, I just wanna satisfy my curiosity that's all.
