I would say a minimum of $20 personally, but that ends up being about $10/lb roughly (eek!) so you might drive off your buyer with that price. (So you probably don't want to listen to that advice, lol) I've heard of processed (and USDA inspected, which has to be done if you are selling meat instead of live animals) guineas selling for $10 a pound, but again, that's all ready to cook and inspected. Other than that, I've no idea what a live bird should go for.
Jumbo or French guineas are slaughtered between 12 to 16 weeks, sometimes a little later. For regular guinea fowl, I'd say a minimum of 16 weeks, maybe a little longer. Jumbos become gamey at around 20 or so weeks? 22 weeks? Something like that.