I have a computer from work that operates with an air card.

Now, at home, I would like to have a lap-top or notebook style computer with an air card. How much would a monthly charge be to have one (internet charges)?

Are we talking a wirless net connection? If so, and you have cable or DSL connected service, there is no additional fee. All you need (here is where the cost comes in) is a wireless router connected to your Cable/DSL modem and a wireless LAN card in your computer.
Many laptops come with them installed. A desktop will probably need a wireless card installed. Try this link:

http://www.wlana.org/learn/educate.htm

I do have Cable modem. But what if I wanted to use my aircard.. or wireless connection... at work? Will I be able to use the same account as my home computers cable modem?

Doubtful. Your work connection is through your employers net service.