I think maybe what you're looking for is
###,##0.00
As per http://msdn.microsoft.com/en-us/library/system.windows.forms.maskedtextbox.mask%28v=VS.80%29.aspx:
Masking element "#" - Digit or space, optional. If this position is blank in the mask, it will be rendered as a space in the Text property. Plus (+) and minus (-) signs are allowed.
While "9" is also a "Digit or space, optional", it says nothing about it being rendered as a space if blank, which leads me to believe that it is rendered as a 0 if blank.
If you want the user to be able to type in a number with no decimals then you should use
###,##0.99
Based on the above interpretation of the masking element descriptions, this means that the user doesn't HAVE to type anything after the decimal, but in the text rendered, they will still see the "_.00" decimal places.
Give it a shot anyhow.
Also, I could be getting this completely wrong, so I would also attempt the following variations if that doesn't work:
###,##0.00
999,999.00
###,###.00
###,###.##
999,999.99
Alain