According to the article, the average YC company raises $700k in seed funding. Assuming your $3M pre is correct, that would be about 18% total and 4% for the $150k investor.
The question is, does a quick and easy $150k delay fundraising until a company is looking at a $6M pre? For the super hot ones I bet it does. For those startups that are more worried about not getting early traction they probably won't delay fundraising.
So if we are going to throw a number around... my fuzzy math says 2% of the hottest companies and 4% of the others.
The first post-YC fundraising is usually made up of convertible notes. So in your example, that $700K at $3M pre would not trigger the conversion of Start Fund's note, which would instead convert at the valuation of the next round of fundraising.
The question is, does a quick and easy $150k delay fundraising until a company is looking at a $6M pre? For the super hot ones I bet it does. For those startups that are more worried about not getting early traction they probably won't delay fundraising.
So if we are going to throw a number around... my fuzzy math says 2% of the hottest companies and 4% of the others.