how to make user enter valid amount?

hi guys, currently i used FilteredTextBoxExtender to make sure user can only enter number into textbox. how can i only allow one point(.) to be keyin?

for an instance, 12.00 is allowed. but 12.0.0 is not allowed?

please advise. TQ

<cc1:FilteredTextBoxExtender ID="ftbe_turnover" runat="server" FilterType="Numbers"
TargetControlID="txt_turnover"></cc1:FilteredTextBoxExtender>