I need help with creating the correct .NET validation control for a text field

I have an edit control on an .aspx page. I need to validate the control
so that it matches the way the data type is defined in the database. In the database the column name ReportedPurchasePrice is defined
as follows: ReportedPurchPrice (decimal(19,4),null)

This value that is stored in this column is obtained from the following edit control in the .aspx file:

<TR>
<TD width="<%=leftMargin%>"></TD>
<TD>
<asp:textbox id="ReportedPurchPrice" Width="250" CssClass="Normal" runat="server"></asp:textbox></TD>
</TR>


Basically I need help writing the correct validation control so that it correctly validates the value defined in the database. In this case:
ReportedPurchPrice (decimal(19,4),null)
...