Behaviors and Bindings

I am creating a behavior to extend the functionality of the Entry control.

Here is the code for the behavior:

public class EntryExtendedBehavior : Behavior<Entry>
{
    public static readonly BindableProperty ValueProperty = BindableProperty.Create<EntryExtendedBehavior, int>(p => p.Value, 0, propertyChanged: OnValueChange, defaultBindingMode: BindingMode.TwoWay);
    public static readonly BindableProperty FormatProperty = BindableProperty.Create<EntryExtendedBehavior, string>(p => p.Format, string.Empty, propertyChanged: OnFormatChange);

    private static void OnValueChange(BindableObject bindable, int oldvalue, int newvalue)
    {
        var e = bindable as EntryExtendedBehavior;

        if (e == null)
            throw new Exception("ExtendedEntry.OnValueChange bindable == null");

        e.Value = newvalue;
    }

    private static void OnFormatChange(BindableObject bindable, string oldvalue, string newvalue)
    {
        var e = bindable as EntryExtendedBehavior;

        if (e == null)
            throw new Exception("ExtendedEntry.OnFormatChange bindable == null");

        e.Format = newvalue;
    }

    public int Value
    {
        get { return (int)GetValue(ValueProperty); }
        set { SetValue(ValueProperty, value); }
    }

    public string Format
    {
        get { return (string)GetValue(FormatProperty); }
        set { SetValue(FormatProperty, value); }
    }

    protected override void OnAttachedTo(Entry entry)
    {
        entry.Focused += OnFocused;
        entry.Unfocused += OnUnfocused;
    }

    protected override void OnDetachingFrom(Entry entry)
    {
        entry.Focused -= OnFocused;
        entry.Unfocused -= OnUnfocused;
    }

    private void OnFocused(object sender, FocusEventArgs e)
    {
        Entry entry = sender as Entry;

        if (entry == null)
            throw new Exception("ExtendedEntry.OnFocus sender == null");

        entry.Text = entry.Text.RemoveFormat(Format);
    }

    private void OnUnfocused(object sender, FocusEventArgs e)
    {
        Entry entry = sender as Entry;

        if (entry == null)
            throw new Exception("ExtendedEntry.OnUnfocused sender == null");

        var unformattedValue = entry.Text.RemoveFormat(Format);
        Value = Convert.ToInt32(unformattedValue);
        entry.Text = unformattedValue.ApplyFormat(Format, Value);
    }
}

And this is the xaml code that binds to the properties:

<Entry Grid.Column="1" Grid.Row="1" Text="{Binding PurchasePrice, Mode=OneWay, Converter={StaticResource fieldConverter}, ConverterParameter=Dollar}" Keyboard="Numeric" TextColor="{StaticResource textColor}" >
    <Entry.Behaviors>
        <b:EntryExtendedBehavior Format="Dollar" Value="{Binding PurchasePrice, Mode=TwoWay}" />
    </Entry.Behaviors>
</Entry>

The Format property works correctly but the Value property seems to be completely ignored. I'm assuming that it's something stupid that I'm just missing but I can't figure it out.

A little background; the entire reason for this binding it to attempt to make the Entry control only bind to the backing VM property when it looses focus. doing a normal two way binding caused it to call back to the VM on every keystroke. This being a financial app, I don't want to recalculate that often.

Thanks