Hi Stalker,
This is great idea, good luck.
I used to own a Baron and roughly I would allow $3 per cylinder per half hour for maintenance costs. Clearly a lot of variables involved (engine management skills, climate, high altitude ops, night v day, APU installed or not etc..) but $3 always seemed around the mark.
For a flying school though you might consider something closer to $4 per cylinder/half hour.
Hope it goes well..