I think the
PromptCharacter
attribute is what you are looking for. Below is the example:
<ajaxtoolkit:maskededitextender targetcontrolid="TextBox2" mask="9,999,999.99" messagevalidatortip="true" onfocuscssclass="MaskedEditFocus" oninvalidcssclass="MaskedEditError" masktype="Number" inputdirection="RightToLeft" acceptnegative="Left" displaymoney="Left" errortooltipenabled="True" promptcharacter=" " xmlns:ajaxtoolkit="#unknown" />