If you do not stay in the Czech Republic for long but still want a cell phone, a prepaid card is the right choice. They are offered by all providers and you can choose whether you only want the SIM card or a phone as well.
To get a prepaid card you usually have to pay around 200 to 500 CZK. A part of that money is your starting credit. If you want a phone as well you will have to pay about 1,200 CZK for a very basic one.
Prices per minute range from 3 to 7 CZK for calls within the Czech Republic, and SMS messages cost between 1 and 4 CZK. As a rule of thumb you can keep in mind that the more you use your prepaid account the cheaper your tariff will become.
You can also get a contract with one of the mobile providers, but they usually run for a minimum of 24 months. The tariffs are not much different from prepaid solutions, however you will get a much better cell phone. Very often special bundles are included, such as free minutes or SMS bundles. Be aware that offers are usually stated without VAT.