For a consumer product, $5-9/month is probably a good price point depending on the product. To a business, $20 per month is a great starting price. Businesses have a different set of demands than a consumer. The value of Basecamp has definitely proven itself to be worthy of that rate. There are "free" services out there that do similar things, but to a business they probably feel more secure paying for a service, knowing that they'll be around for the foreseeable future.