One thing to keep in mind is that Diablo III is a global game, available in multiple regions around the globe. And, across those regions, the decimal style varies.
In the US, for example, decimals are identified with periods (ex: 1.23). Whereas in a lot of places in Europe, decimals are identified with commas (ex: 1,23). This means that a number like one million would look a bit different in each of those regions. In the US, it would look like "1,000,000" and in Europe it would look like "1.000.000".
So, which decimal style do we use? That question is actually one of the main reasons we chose not to include commas initially, since it could be confusing to players in other regions outside North America. In the end, we just decided to go without separators, but we didn't fully realize what kind of impact not having breaks would have on the user experience (and, yes, that's our bad).
That said, we agree that not having any method of breaking out values in the auction house isn't ideal, so we're currently discussing how best to address the issue (hopefully without creating other issues for players in other countries). Sure, we could just use commas since that's what's already available, or we might just do something more universal likes spaces.
Edited by Lylirra on 11/5/2012 10:10 AM PST