It's pretty common for most of us to free feed our Guineas and let them eat what they want/need... but adults Guineas will do fine on 1/2 cup of 16% layer feed per bird, per day, plus some free range time for bugs and greens. I don't know how much feed that equates to per bird per month, since I have 125+ birds... but 1/2 cup of layer feed weighs about 3 oz, give or take a little, so you can do the math if you want and get a better "guesstimate".
Some Guinea keepers/breeders feed a little higher protein game bird feed or higher protein layer feed in the winter months and also add some scratch, rolled or cracked corn to bump up the fat and calories to help keep them warm in the winter (I personally use sweet feed/wet cob as a supplement/treat, and switch to a 50/50 mix of 24% game bird and 16% layer pellets thru the coldest wettest months). I also feed them fine stem, leafy alfalfa hay free choice year round for them to pick thru, and keep them occupied.
It's best not to overfeed them corn, scratch or sweet feed and try to to keep their protein intake at no less than 16%. You want healthy birds that can stay warm, but not overly fat birds that can develop fatty organs, health issues and fertility/reproductive problems.