$10 a "keeper" image would be very fair.
This would, I suspect, put it in the ballpark of ReyRey's recommended fee.
If you go flat rate (let's say $300, which is a bargain for them) make sure you have a cap on how many you supply them with in hi-rez.
You need to control your post-production commitment as well. If you are just shooting and then handing them the files from the camera, then the above fees are fair. If they expect you to crop and do levels and clone wall stains, then you need a fee for that too. Believe me, post production can take up more time than shooting sometimes. Specially if you are shooting fast, which it seems you need to here.
In the end, what would YOU feel good getting, money-wise? You want to feel no resentment at the end of the day. For a 20 year old dude, $300~500 should help keep the resentment-wolves at bay, I would think!
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