It's not entirely clear what is causing the bug yet, but as of this writing (and for the past half hour or so), paid Android apps are not appearing on Google Play — either on the web store or on the Android client. As near as we can tell, it's mostly a listing problem: for example, the "Top Paid" section is gone, but paid apps that are editorially chosen by Google are still showing up. We also know that direct links to apps within the app store work — but only if you already have the link, as searches for paid apps don't list them in the results.
More troubling, the Android Google Play client is also experiencing issues, when you are actually able to find a paid app, attempting to purchase it results in an error. This is the second major Google Play bug in as many months and we're sure that the good people in Mountain are working hard to make everything right — we've reached out to Google for comment on when we can expect a fix and will let you know what we hear back. One Android developer has let us know that Google has confirmed the issue is on its end and is working on a fix.
Update: We might not be entirely out of the woods just yet, but we are seeing some paid apps appear again on both the web front to Google Play and within the Android client — in both cases we were able to successfully purchase apps. There are still some gaps in the listings in both places, but it appears that Google is bringing everything back online.