Pricing is very dependent on which services exactly you plan to use. There is a pretty decent pricing calculator for google cloud services here:
Details about app engine specific service fees can also be found here:
Generally you would pay for the number of instances selected to host your app. Then you have to pay for any network traffic generated by them and any cloud storage you require (as I said before there are other services you might use, which will add costs).
As far as I know you can easily define a "spending limit" for google's app engine, which makes it very easy to stop spending money at a certain point. There are also a minimum number of resources provided to you before you have to pay anything (so you can for example host a low traffic website for free). If traffic suddenly picks up, the site may eventually go offline after you reached your max. resource limit, though.
The main difference between google (or amazon) cloud services and the dedicated hosting providers is that with google (and amazon) you pay for what you use, while with the others you generally pay for what you signed up for (to be honest I'm not too familiar with their business model - so correct me if I'm wrong).
The general idea when using google (or amazon) cloud services is that you can always easily scale up (and down!) as required.