$500k is obscenely low for most companies, but not all. Are you building a lemonade stand?
An old expression is "any fool can sell a dollar for 90 cents." Your willingness to take such a low amount of money for a large amount of equity and control says a lot to future investors, and the news isn't good.
Can I now also invest $400K for the other 80%? (I'd never see it again because with 0% of the company left to sell, you have zero financial reason to stay involved in the company).
In the US, a modern day rule of thumb is $10M post money valuation for your first (seed) stock offering. Most states nudge you toward 10M shares for a corporation, the math is pretty simple for $1 a share.
If you can legally revise that sale to maybe 1% of your equity, or cancel the old corporation and create a brand new one where that money goes into the new one at 1%, it'd be perfectly fine with most investors.