On average you will pay 4 times more to maintain the main in-house than with Google simply because many people never factor the hidden costs of mail, especially the costs of time for the administrator. With Google Apps, you would have the perfect tools to help your coop coordinate using Google Calendar, Google Docs, and Google Gmail all in your brand. Have a shared event, create a Google calendar. Need to develop a document with a team, use Google Docs. All without sending email documents back and forth.
I guess why chance that your server will go down or your administrator won't be able to help you or even the added expense of all the new hardware, software, and bandwidth. Google is the fastest and most progressive email solution available today. We have help many organizations and would like to help you. All this can be done efficiently with our migration team. Let us know how we can help you make this happen.