How do the upgrades work?
Solution 1:
It applies immediately. You'll see in the investors menu that the Bonus per Angel is increased by 2%. The upgrade is reset (just like all upgrades) when you reset the game to get more investors.
I am not sure what the percentage bonus is before you buy it, but I think it is 5%. So you'll get from 5% bonus to 7%. That means the minimal amount of investors to purchase this upgrade is 350.000. (350.000 * 5% = 1.750.000 % bonus, 250.000 * 7% = 1.750.000% bonus)