You can get a cheap phone here for around $30. But, you can also just remove your sim card from your US phone and get a Pamama sim. Calling the US is very cheap using a prepaid sim. I have Skype but never use it to call home because it's actually cheaper using my local Pamama sim (C&W aka MasMovil).
If you live in a city Copa serves, you have to pay the taxes to travel to the US from Panama. You can also just jumpseat on Copa and avoid the taxes, but this requires you get approval 48 hours in advance. Continental will let you jumpseat, but taxes have to be paid.
Good Luck