The lay place markets have very low liquidity. Also the place prices that go on betfair can be much much higher. I have seen prices of $5 on the totes and up to $50+ on betfair.

If your serious about laying then check the betfair prices first to see what that $6.50 winner really went for.

Personally I think your better off sticking to the win market as the place market can be very frustrating with the low / no liquidity.

As far as what strike rate you would need. You would need to hit at least 17 out of the 20 to not place in order to show a possible profit. That is based on your $6.50 amount. ie. $5.50 lost on each placing so you would lose $16.50 is 3 selections ran a place. Even then after commission on the winning bets you are probably showing a loss. So you really need 18 out of 20 to make a profit.