Just completed my trip, visited those 6 countries and survived with no harm ;-)
A few remarks about border fees and stamps:
1) Guatemala, entry & exit: no fee; border stamps: yes
2) Honduras, entry $3, exit $3; border stamps: yes
3) El Salvador: entry & exit: no fee; border stamps: none
4) Nicaragua: entry $10, exit: $1 ; border stamps: yes
5) Costa Rica: entry & exit: no fee; border stamps: yes
6) Panama: entry & exit: no fee; border stamps: entry-yes, exit- no
El Salvador, although not the richest in the area, seems to have the most advanced border control ( magnetic scanning of passports and no stamps).