Forms Data Controls :: Click Button To Increase The Price With 1.5 / Getting Incorrect Values?

my web Form contain Gridview : when i click the button woth ID="IncreaseButton" , i have problem is : The Initial price = 334,7 now When i click button to Increase the price with 1.5 , i got this value :

3347000,0000 wich incorrect

this is the code
<asp:GridView id="ProductsGridView"
DataSourceID="ProductsDataSource"
DataKeyNames="ProductID"
AllowPaging="True"
SelectedRowStyle-CssClass="selectedRow"
CssClass="gridView"
OnRowCommand="ProductsGridView_RowCommand"
AutoGenerateColumns="False"
runat="server">
<RowStyle BackColor="#F7F6F3" ForeColor="#333333" />
<Columns>
<asp:BoundField DataField="Productid" HeaderText="Product ID" />
<asp:BoundField DataField="Name" HeaderText="Product Name" />
<asp:BoundField DataField="ProductNumber" HeaderText="Product Number" />
<asp:TemplateField HeaderText="Price">
<ItemTemplate>
<asp:Label ID="PriceLabel" runat="server"
Text='<%# Bind("ListPrice") %>'>
</asp:Label>
<asp:Label ID="Label2" runat="server"
Text='<%# Bind("ListPrice") %>' Visible="true" >
</asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField>
<ItemTemplate>
<asp:Button runat="server" ID="IncreaseButton"
Text="Increase Price 5%"
CommandName="Increase"
CommandArgument="<%# ((GridViewRow) Container).RowIndex %>" />