How to make the last two numbers typed into a asp:textbox decimals?

Hello Experts,

I have an asp:textbox with the following regular expression validator:

<asp:RegularExpressionValidator ID="rev_case_exp_cost_txt" runat="server" ControlToValidate="case_exp_cost_txt" Type="Integer" Operator="DataTypeCheck" ErrorMessage="Invalid Format" ValidationExpression="^\d{1,11}(?:\.\d\d)?$"></asp:RegularExpressionValidator>

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How can I get it to automatically make the last two numbers entered as the decimals?  For example, if the user types 300, it should show 3.00 after they tab out of the field - or 50000 would be 500.00.  

I am using asp.net and c#.

Any help would be greatly appreciated.

Thanks,
J