ASP.NET - System.FormatException: 'La cadena de entrada no tiene el formato correcto.'

<label>Marca</label>

                   </div>

                   <div class="form-group">

                       <asp:DropDownList ID="ddlMarca" runat="server" DataSourceID="SqlDataSourceMarca" DataTextField="Marca" DataValueField="Marca" Width="184px" AutoPostBack="True">

                       </asp:DropDownList>

                       <asp:SqlDataSource ID="SqlDataSourceMarca" runat="server" ConnectionString="<%$ ConnectionStrings:SistemCSTConnectionString %>" SelectCommand="SELECT [Marca] FROM [MARCAS]"></asp:SqlDataSource>

                   </div>

                   <div class="form-group">

                       <label>Categoria</label>

                   </div>

                   <div class="form-group">

                       <asp:DropDownList ID="ddlCategoria" runat="server" DataSourceID="SqlDataSourceCate" DataTextField="Categoria" DataValueField="Categoria" Width="184px" AutoPostBack="True">

                       </asp:DropDownList>

                       <asp:SqlDataSource ID="SqlDataSourceCate" runat="server" ConnectionString="<%$ ConnectionStrings:SistemCSTConnectionString %>" SelectCommand="SELECT [Categoria] FROM [CATEGORIAS]"></asp:SqlDataSource>

SqlCommand sqlCmd = new SqlCommand("ProductosCreateOrUpdate", cn);

            sqlCmd.CommandType = CommandType.StoredProcedure;

            sqlCmd.Parameters.AddWithValue("@id", (hfProductoID.Value == "" ? 0 : Convert.ToInt32(hfProductoID.Value)));

            sqlCmd.Parameters.AddWithValue("@descripcion", txtDescripcion.Text.Trim());

            sqlCmd.Parameters.AddWithValue("@idmarca", Convert.ToInt32(ddlMarca.SelectedValue?.Trim()));

            sqlCmd.Parameters.AddWithValue("@idcategoria", Convert.ToInt32(ddlCategoria.SelectedValue?.Trim()));

            sqlCmd.Parameters.AddWithValue("@existencia", Convert.ToInt32(txtExistencia.Text.Trim()));

            sqlCmd.Parameters.AddWithValue("@precioA", float.Parse(txtprecioA.Text.Trim()));

            sqlCmd.Parameters.AddWithValue("@precioB", float.Parse(txtpreciob.Text.Trim()));

            sqlCmd.Parameters.AddWithValue("@precioC", float.Parse(txtprecioc.Text.Trim()));

            sqlCmd.Parameters.AddWithValue("@precioD", float.Parse(txtpreciod.Text.Trim()));

            sqlCmd.ExecuteNonQuery();

            cn.Close();