Hello Annet, the first place to get started with fish farming business... Is by learning from the researchers/scientists from this platform . They  will help point you to the right direction. To get started here is the link  http://agromarketday.com/course_details/18-cage-construction?path_id=18

The price of a cage depends on the size of the cage ie 4*4. Please check out this link for the various prices depending on the cage size  http://agromarketday.com/market/inputs?category_id=2