How much would it cost or how difficult would it be for this kind of script/app?

I'm pondering on the idea of an app or script that allows users to register, then with a click of a button from their dashboard, the app begins opening Facebook pages, liking them (or liking specific posts), then moving on to the next page to like. In the mean time, users would rack up points. Kind of like AddMeFast, but automated. (And I know there are AddMeFast bots, but I'm inquiring about a web app or script that would be the whole package.)

I think your idea can work, but you're better off just posting in 'Hire a Freelancer' for an app and then telling them your requirements. I mean if there's nothing like this about, what's stopping someone just cutting you out and doing this themselves?
Don't let your brainwave go to waste!

I think most cost effective solution would be a mix of c#/vb/(insert you fav .net lang here) , asp.net, and imacros.