host my own or get an ISP to host?

Hi,
Ive been devloping intranet applications for about a year now for remote employees. However there is now a requirement for users (suppliers) external to the company to gain access to the system. To do this at the moment they have to dial into our network. Each time a new supplier wants to gain access they have to be set up to dial in, this is very time consuming. I have considered getting an ISP to host the "extranet" for us but the system needs to be intergated into our current intranet (same server) and I am concerned about us losing control of the data. How would we go about hosting our current application and making it available on the WWW. What sort of costs are involved?