I think there is always a market for business apps that deliver value. We're a startup ourselves and pay for Basecamp and Highrise - sure there are free alternatives but BC and HR are intuitive and easy to setup and when you have non-techies included in your business those things are important.

Someone mentioned Salesforce - also check out Netsuite.

You should determine if you're 100% pay (with maybe a 30-day free trial) and primarily directed towards SMBs or freemium model directed to users and businesses. If freemium you can expect a 1-5% uptake on the paid services.

Cheers