I would say cheaper to build your own. Components for a C&C cage can be purchased off of Amazon (maybe with an extra set of connectors to be sure of having enough), and they're not that difficult to construct. The hardest part is figuring out how to cut the coroplast and tape it to the size you need. Or you can order the coroplast (and the rest of the cage components too if you like) from the Cagetopia store (link at the top of the page).
Two guinea pigs... minimum of a 2x3 C&C (8 sq. ft.) but bigger is better. And if you have two boys, minimum of a 2x4 C&C (10.5 sq. ft.).