What is the ideal and more realistic outlook of a client billing/management system?

- HelpDesk/Ticket System
- Client Management (All the normal stuff included)
- Billing Management (Reoccurring payments, invoices, etc.)
- Live Support?

I am asking this as to this is 1 of many projects on my mind right now. Instead of going out and purchasing a license, I am pretty well in depth with PHP and MySQL so I decided to take a shot at something myself.

If the product is successful and well stable for intense usage, what types of realistic prices would you like to see on a such item?