What they are.

How they work.

A network card (Ethernet card):

Price: $10 - $70

This is standard equipment in most offices.

ADSL modem (xDSL modem):

Price: $??
Some details about ADSL, SDSL, IDSL
BellSouth currently does offer this in Daytona Beach.
Pricing from BellSouth
Regular dial up phone modems:

Price: $40 - $130


Cable modems:

Price: $250 - $500
Currently $300 from Time Warner and another $50 for the power supply 

There is two different systems depending on the cable operators network.
The first system is a more modern cable network capable of two-way traffic.
The second system is a cost effective solution for the cable operator in which upgrading their network is not cost effective for two way traffic.