1. They no longer have interest in catering to small businesses. Small businesses are no longer a target. Their focus is to gain market share in the enterprise market.
2. They are feeling so confident that their software will sell, even with a 100% price increase.
3.Something big is about to happen. An acquisition? 37Signals going public?
I can’t see the change having anything to do with cost. They can easily scale the number of users without a large increase in cost (unless they are struggling with support issues, and the small accounts are no longer worth the trouble).
Why do you think they doubled the fee?