I often see people asking about a cheap cage for guinea pigs. I made mine for under $25 (and since this photo was taken I've expanded it to almost twice the size in this photo and used leftover materials.) Store bought cages are almost always twice this expensive.
Store-bought cages don't give guinea pigs enough room. (Guineas need anywhere from 4 square feet to 7 square feet PER PIG so I know my cage still is on the border of small, and it's twice what you see in this photo.)