calculate your costs, model, wardrobe, accesories, makeup, location, photographer, etc. and start from there.
say if the photographers are already being paid a monthly salary and have a lot of idle time, and your only cost is $500 for the model then about $1000 should be more than ok,
if you have a lot of production costs just mark up enough after costs so it's worth it for you to take the jobs
again, how many sets are you thinking about? how about large order discounts?