We haven't posted our border post yet for El Salvador. It's about the same as the rest. We took the La Hachadura crossing.
Simple, but slow.
Leaving Guatemala: no costs outside of photo copies. A couple extra 'friendly' helpers who simply don't take no for an answer. So you just end up with a puppy following you around.
Entering El Salvador: Migration officer will scan your passport, but no stamp in the passport since you have the CA-4 already.
If you're Jayne, the customs man might buy you an ice cream. Paperwork takes awhile, and was FULL of mistakes. Check everything over. But no charge for any of the paperwork.
Must buy the "insurance" for the bike at 10$.
Also had to pay 5$ for the "community road improvement tax". This seemed more like a gringo tax, but they gave a reciept, and the cop wasn't letting us go anywhere without paying it. Other borders may not have this, or may have a different community tax.
Have a great time in El Salvador!