Here is what I had to pay when I entered each of these countries (July 2009).
Guatemala: ~$1 (I forget exactly)
El Salvador: free (and they don’t even stamp your passport)
Nicaragua: $10 [$7 to enter with lots of paperwork, then $1 to enter the border area in Sapoa at Costa Rican border (weird), then $2 to exit the country, paid only in Cordobas or US Dollars (so don't change your money until you get to the other side – I made that mistake)]
Costa Rica: free (but long lines at Penas Blancas, coming from Nicaragua)
Panama: $1 (for a stupid passport sticker you buy from an unofficial-looking guy who hangs out by the immigration area, but apparently you need it)
You get 90 days of travel within the four countries of “centroamerica:” Guatemala, El Salvador, Honduras, and Nicaragua.