I assume you are asking how much you should charge to design a business card. The answer to that is:
your hourly rate * the number of hours it takes you to complete the job
The key is to figure out your hourly rate. That is:
annual revenue $ / # BILLABLE hours you work per year
Your annual revenue is all the money you need to bring in to cover all expenses and leave yourself a profit for your salary.
annual revenue $ =
desired salary + insurance + equipment + software + office supplies + etc
Your billable hours are the hours you can actually bill a client for doing work. This will NOT be 40 a week.
annual billable hours =
(40 - non billable hours) * (52 - (vacation + sick time))
Non billable hours are all the hours you spend running the business rather than actually doing client work. High end firms hope to get 75-90% billable hours out of their billable staff. On your own, expect that to be closer to 50%, as it takes a lot of hours to just keep the business going (accounting, billing, marketing, travel, tech support, training).