Guineas and chickens??
Guineas are cocks and not roosters. In my opinion $20 is too much for a male guinea. I also think it is a very bad idea to keep just one guinea since they are a flock bird. It is quite possible for a lone guinea to cause harm to your chickens especially if confined in a small area.
Sorry if that is considered cruel or something; I would pay maybe 5 bucks for a male for the dinner table, but not for a flock of chickens.
If you're interested in guinea, the best bet is hatchlings in spring. That way, they have brooder buddies to pal with and won't try to start mayhem with the chickens, which they can indeed do. I had to ship my males off to my aunt because they were being very "jerky" to my rooster, and they got a little too territorial over hens that weren't guinea hens lol
Guineas can be kind of hard to handle if you don't have a lot of space, too. Ornery buggers lol