Honestly, guinea pigs would be much happier with the company of a fellow guinea pig(s) than humans. As long as you regularly clean their cage (spot cleaning fleece daily, then doing a full cage clean once a week will do, sometimes twice a week depending) and feed them their necessary foods (good quality hay, variety of veggies, and good quality pellets without seeds, grains, or colored bits) they will be fine.
Something else to keep in mind, if you rehome them, you can't guarantee the next people that adopts them will give them more attention, or treat them well.
As for cost per month, I buy fleece instead of paper bedding, so there's an initial cost and that's it. I buy a variety of veggies that cost an average of $10 a week. As for pellets, I purchase the 10lb. bag of timothy pellets from KMS Hayloft for $22.95 (includes shipping) and then freeze them so they're good for about 6 months. The hay I purchase is also from KMS Hayloft - I buy the 45 lb. box for $69.67 (includes shipping) and it lasts me a good 6 months for my two boars.
That's an average of $55.44 per month, or $665.28 per year, to give you an idea.