Make it region-specific.
It's frankly pathetic that this hasn't been implemented already.
There are three gameplay regions for Diablo III:
Many countries within those regions use different decimal styles, so unfortunately it's not as simple as making it "region-specific." Also, making a change apply to only one region and not another isn't always ideal or an available option.
Now, like I said, we agree it'd be nice for players to break out values in the auction house (at the least the values that they enter themselves). In the end, we may just add commas, since that's what the service already uses in a lot of places. We're also considering other options (like spaces) that are more universal.
If regional isn't a good option, you could try making a it a language setting. I mean periods commas or spaces dont work for people maybe they will pick the option with some other random symbol. It could be fun. 1<000<000<000>00