How much is agates worth?

That depends on a lot of things. Looking at agates posted for sale on google, I see some for $15, and others for several thousand dollars. It looks like it depends on size, quality, cut, imperfections, etc. What I would do is compare the agates that you have to the ones that you see online for sale, and then determine what price you could get. The other option, but it might cost money, is to go to a gem expert and have your agate appraised.