$50-100 is typical for a JP or other official performing a basic service, though I think more is appropriate if they are taking time they would otherwise have off to perform the service. I would double check that they don't already have a "donation for services" request set up, when I got married the officiant (a civil servant) asked for a fee that went directly to a pet civil project of theirs in the town.
It really depends on the region. In CT My friends paid $300+ for their JPs. I got married in VT by an adorable 93 year old that charged us "whatever you think you two can afford."
Not familiar with using a JP as so far we've had a church wedding and a courthouse wedding.
I liked the courthouse one best - buy the license and then wait your turn and it's free! Plus they have a nice little room set up sort of like a chapel with an arbor and other decor - I wasn't really expecting much but was nicely surprised.
The church one was over the top with all the trimmings and was a years worth of work and expense.
The next wedding may be a JP and a destination so watching for your responses.