What would be considered a good return on an investment of 50G with pay out in 20 years...10%, 15?

If by "payout" you mean receiving nothing before 20 years then I wouldn't go for it at all.

11% is about average return on the stock market, although with the market being slower, returns could be lower. Mutual funds wouldn't give that kind of return, too many fees involved. If you could find a property for $50K, renting it out could give good return, but without doing the math, my estimate for rent would be $500/month, if you wanted it back in 20 years

Presuming you are an average investor, probably 7-8%. Higher rates require very significant skill levels and are a thing of the past mostly. The stock market is valued at around the 7-8% range over twenty years in aggregate, although islands of value exist. The bond market will provide in the 6% range and real estate will probably come in below the rate of inflation.

Regardless of the amount of the investment, a 7% interest compounded annually would provide a 100% return after 10 years, 300% after 20 years.

In other words at 7% interest a $50,000 investment would be worth $100,000 in ten years and $200,000 in 20 years.

You want to diversify your money. Don't put it all in one investment. And as risk always increases with increased return on your investment, you probably don't want to invest in anything that promises above 7% or so. Anything that promises 10% or higher is too risky for most investors.

15% would be considered an excellent return. 10% a good return assuming that inflation did not rise above 3% in the mean time.