That's a good point -- ensure you get the same class of shares as the founders. Yes, sometimes even founder shares get diluted to worthless, but it only happens when the alternative is the company folding, in which case the shares would be worthless anyways.

Here's the math I do:

share of company * realistic potential valuation * chance of hitting it * expected dilution.

chance of hitting it ~= 3% expected dilution = 50%

So using your example:

0.0005 * 1B * 0.03 * 0.5 = $7500