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.
You'll need to contact your local planned parenthood to find out for sure, the prices vary from state to state. For the most part if you are under 18 and don't have a job it won't cost you anything, or it will cost you very little.
My friend and his chic had sex and the condom broke so she went to PP and had to pay like $32 for the 5 day pill, but the morning after pill is prolly a lot cheaper. Good luck. Oh yea, this is in Northern Michigan so it might be different somewhere else.