Yeah, also, I good plan, ready for loses, staff, sales plans, etc, is just as important.
The startup cost will cost a bit more:
3rd party payment processor(Paypal, 2checkout)
Helpdesk(Osticket, Perldesk)
Billing System(PHPCoin, Whoiscart)*
Support: Outsource(Ehostcare) or Normal staff members
* Denotes Optional