C&C cages are generally cheap to build.
Cubes can be found on ebay for around £20-25 and Correx can be found for £15-20 if you look in the right places. (I got some neat links from my lovely pals here.

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They may look fairly big but your pigs will benefit from the space. Lack of space means they have a very high chance of fighting and they can cause serious damage to each other and not to mention to their relationships.
If you're not careful, they can fight to the point where they simply cannot be together anymore and then you end up with one cage per Guinea Pig, having to have them close together so they can see and chat to each other and that ultimately means you lose even MORE space. Not to mention money!
So Building a decent sized C&C cage from the off is normally the best idea.
http://www.guineapigcages.com/index.htm
Check this site out, too. It'll be a big help.
Maybe I'm insane but I go without so that my boys can have the space they need.
I have to climb over my bed to get to things in my room because their cage blocks most of my floor.
Believe me, these little guys are worth every sacrifice.
Don't get me wrong. Store bought cages are great! I'm buying one myself soon.
But they need to be big enough and they can be rather expensive.
C&C cages are generally cheap and great because you can expand them or even shrink them if you need to for any reason. (Like if you want to split a piggy up from the rest because he/she is sick. things like that.)