how much does a CBS app cost

Depends on the App. Most of them are Free, including the CBS, CBS News, CBS Sports, 48 hours and 60 minutes apps.

Though some will offer subscriptions and in-app purchases to access more content within them.

You can check pricing and details in the App Store.

https://itunes.apple.com/us/app/cbs-full-episodes-and-live-tv/id530168168?mt=8

https://itunes.apple.com/us/app/cbs-sports/id307184892?mt=8

https://itunes.apple.com/us/app/48-hours-for-ipad/id449948632?mt=8

for many other cable or channel apps the apps are free, but to get access to content you have to give them your cable account info....basically prove 'yeah, I am paying for this, now let me stream on the app for free'

I believe CBS All Access app to unlock ALL episodes and shows is a monthly subscription price of $6.99/month, no cable subscription required.