Both females (sows) and males (boars) can be castrated, but you’ll obviously only need to castrate one gender or the other in order to prevent being overrun by babies. Neutering costs vary, but typically a neutering operation will cost about £50 per guinea pig. Sows can be castrated nowadays with injections, but boars will still need the full operation. It’s worth bearing in mind that any operation is both stressful and dangerous for your guinea pig, so if it’s possible to keep guinea pigs in gender combinations that a.) don’t necessitate neutering, and b.) leave no guinea pig on their own, then that might be the best good option.
Guinea pigs can be neutered to prevent them reproducing
Although baby guinea pigs are very cute, it's worth bearing in mind that mothers can find it a very stressful experience, and there can be quite severe birth complications. If you want to add some more guinea pigs to your group, you might want to consider adopting some. There are hundreds and hundreds of guinea pigs of all breeds and ages waiting for new owners in shelters all across the country, and they'd love to come and join you.