Hi,
If your objective is only to filter a valid character input...
try this if could help:
<ajaxtoolkit:filteredtextboxextender id="FilteredTextBoxExtender1" runat="server" xmlns:ajaxtoolkit="#unknown">
TargetControlID="TxtPrezzoUnitario" ValidChars="0123456789" FilterMode="ValidChars" />
</ajaxtoolkit:filteredtextboxextender>
Hope this could help.
Regards,