they aren't as cheap as you would think. The forum link will tell you more....also expensive is relative to how much money you have and what else you want to spend it on....they you have to think about vet bills and so on. Even when its cold and wet and they aren't running around in the garden, you will need to look after them and not just leave them shut away. if you keep them in a garage or shed when its cold, they will need light as they won't be happy in the dark all day, they may also need some winter heat.
I used to like having a hamster because he lived indoors with me and I used to let him run around inside my shirt and jumper while I was sitting revising...my mum wasn't so pleased as he chewed holes in my clothes and took them for bedding lol. Guineas live outside so are less easy to interact with, especially if you have to keep them away from the cat and dog, WHICH YOU WILL, the risk to the guineas is too great.