Paul Walnut makes some good points. I'll add a few thoughts of my own.
If this a public re-enactment? In other words, do they invite the public to come out and watch? Or is it a closed set, such as for a movie or documentary.
I'm guessing it's a public or semi-public event, in which case anyone with a camera could come out and shoot pictures. They are then paying you $100 so they can get guaranteed access to your pictures and the ability to use those pictures.
Are they paying you enough? Is it fair? Not really, but then again, they are paying you $100 and presumably giving you some access that others don't have. Also, they may or may not be publishing your pictures. Since this a resume builder for you, only you can decide if it is worth it.
What I would not do, though, is offer them any sort of exclusive right to your photos. You want to retain your own ability to use the photos in your portfolio. If they just want a hundred or so nice JPGS that are processed and ready for conversion into CMYK, that's one thing. If they want you to agree not to use the pictures yourself, that's quite another. That's why it is important if this is a public event. If it is, then they don't really have much ability to restrict or limit how you use the pictures.
Personally, I'd try to negotiate photo credits and 25-50 copies of any publication they may use them in. That's far more useful to you than the $100.
Most importantly, I would strongly urge you to treat this like a job paying $10,000.
That means, doing research on the group beforehand. Clearing your day of everything else. Getting there early in the morning and shooting them setting up, getting ready, etc. etc., talking to the re-enactors to find out what roles they are playing (most model themselves after a specific person or create their own persona), asking them how the "battle" has been scripted (who gives the orders to charge, fire, etc. and etc.,) scope out the best vantage points, shoot detail shots before the action happens, shoot portraits before the action happens, etc. etc.
Many times, the re-enactors come in the night before and set up camp with tents, etc. Find out if that is the case and be prepared to show up the evening before to shoot some of that activity.
Why do this? First, to get the best pictures possible and second, to show them that you are a professional, even if they don't pay you like one. If people ask you what you are getting paid, tell them you are doing this as a pro-bono project because you want to help the organization.
After the event, turn around the images as quickly as you can. Give them a great selection of nicely processed images in both full size jpg and web-optimized formats. Make sure you clearly label them as such in their file names, so they don't try to use a 72ppi image in a publication. Give them the images on a USB drive.
You want to leave them with the impression that you are professional and you want them to spread the word around their peers that this person is a real pro and shot the best pictures they've ever seen.
Finally, by treating this as a professional job, you'll get a taste for what it entails. You'll understand better how many hours such an assignment really requires and you'll be better able to gauge 1) if doing this on a professional basis is worth it to you and 2) if the market could ever bear the cost that would be required to make this profitable for you.