A gigabyte plan counts every byte in both directions: the pages, and also their images, scripts, fonts and video. A typical web page weighs two or three megabytes when everything loads, so one gigabyte is a few hundred full pages in a browser, or tens of thousands of small API answers.
To make a plan last, block images and media, ask for compressed responses and avoid fetching the same thing twice. Where the volume is large and the target accepts datacenter addresses, a plan sold by time with no traffic limit costs less.
Read next: How much proxy traffic do I need?