Not sure on those specific models, but here's what you need to do it on an HTC Evo, with a 4.3" screen for only $12. Since the Bionic is similar, I would imagine we will soon see a similarly priced kit for that model. Apple stuff is notoriously overpriced, but even if you don't want to do the work yourself, I know people who have taken their iPhone 4 in and had the screen replaced for under $100, and if you do it yourself it's probably closer to $20 than $100.
Certainly not hundreds of dollars on the phones -- I still stand by my original statement.