Costs vary widely. Different accounts at the same ISP provide different services and different amounts of time online. There is no standard pricing. Your ISP will tell you how to configure your computer to connect. It's not really that hard to do.
Modems come in several different speeds.
Do not settle for 28.8 Kbps if your ISP can handle higher rates. Be careful that your kind of modem works well with the kind that your ISP uses. ISDN requires a special kind of connection with your telephone company. This will cost extra. The advantage is a much higher speed of data transfer than with a modem.