One group I am having transferred has about 3 GB of photos and files, so I opted for the $10/month (actually, $110/year) cost.? Asked for donations and got $5, $10 and $20 donations (via PayPal) to cover the cost, and that's a group of about 50 people.? It is a friendly group, we pretty much know each other.
So with a large group like you have, you might ask for a little cash to cover the cost and I bet you'd be surprised how generous people can be.? Plus it's a LOT more fair to the IO guys and will help keep them in business.
Mike Conder
