Hey MP59402,
Use MaskEditExtender Ajax Control with your TextBox.. Like This
<ajaxtoolkit:maskededitextender xmlns:ajaxtoolkit="#unknown">
TargetControlID="TextBox2"
Mask="9,999,999.99"
MessageValidatorTip="true"
OnFocusCssClass="MaskedEditFocus"
OnInvalidCssClass="MaskedEditError"
MaskType="Number"
InputDirection="RightToLeft"
AcceptNegative="Left"
DisplayMoney="Left"
ErrorTooltipEnabled="True"/>
</ajaxtoolkit:maskededitextender>