Here is the information on the data plans for ICE/Kobli Prepaid chips.
http://www.grupoice.com/wps/portal/kInternet_prepagoif you don't activate a data plan, then you get charged by the KiloByte for Internet use.
If you use the Internet frequently, then it's best to just text descarga to 8888, and let it deduct 500 colones per day. You get 2MB of bandwidth and 350 MB per day.
It renews each day automatically. So you have to text SALIR INTERNET to turn off the package before you leave. Otherwise it will eventually deplete your balance.
You can check your prepaid balance with *1150#