On GoDaddy at least, pricing seems to vary by domain name. beer.app is $1,999.99 while hackernews.app is $16.99. You can check pricing on individual .app domains here: https://www.godaddy.com/tlds/app-domain
Edit: It appears this pre-registration doesn't even guarantee you'll get the domain. It just increases your chances :( I'm gonna pass...
How would it? Godaddy can't buy the domain until General Availability, so they can't guarantee that you'll get it. Someone else could buy it first.
Yeah, this isn't Early Access. :(
We aren't (and cannot) sell directly to end users. Every domain name registrar sets its own price.