I have no idea what Planned Parenthood's official policy is but quite often if you go to a clinic, especially a 'free' clinic, they might give it for free (doctors get samples they can give out at their discretion). You don't necessarily have to be under 18, but being poor helps, and so does asking. And if right now you think you need it, you have 72 hours from the time of intercourse for it to be most effective (although you can take it after that), so I wouldn't let $20 stop you from getting some. And the price actually varies. You can always phone whatever clinics are in your area to find out for sure.