NumericEntryAccessoryVIewEffect.cs

public class NumericEntryAccessoryViewEffect : PlatformEffect
{
    protected override void OnAttached()
    {
        if (!(Element is Entry entry)) return;

        entry.PropertyChanged += EntryOnPropertyChanged;

        UpdateDoneButton();
    }

    private void EntryOnPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == nameof(Entry.Keyboard)) UpdateDoneButton();
    }

    private void UpdateDoneButton()
    {
        if (!(Element is Entry entry)) return;

        if (entry.Keyboard == Keyboard.Numeric) AddInputAccessoryView();
        else RemoveDoneButton();
    }

    private void RemoveDoneButton()
    {
        if (!(Control is UITextField textField)) return;

        textField.InputAccessoryView = null;
    }

    private void AddInputAccessoryView()
    {
        if (!(Control is UITextField textField)) return;
        if (!(Element is Entry entry)) return;

        var toolbar = new UIToolbar(new RectangleF(0.0f, 0.0f, 50.0f, 44.0f));

        var minusButton = new UIBarButtonItem("+/-", UIBarButtonItemStyle.Plain, delegate
        {
            entry.Text = entry.Text is {} text && text.StartsWith("-")
                ? entry.Text.Substring(1)
                : $"-{entry.Text}";
        });

        var doneButton = new UIBarButtonItem(UIBarButtonSystemItem.Done, delegate
        {
            if (!textField.ResignFirstResponder())
                // if resign was unsuccessful, force it
                textField.EndEditing(true);
            ((IEntryController) Element).SendCompleted();
        });

        toolbar.Items = new[]
        {
            minusButton,
            new UIBarButtonItem(UIBarButtonSystemItem.FlexibleSpace),
            doneButton
        };
        textField.InputAccessoryView = toolbar;
    }

    protected override void OnDetached()
    {
        if (!(Element is Entry entry)) return;

        entry.PropertyChanged -= EntryOnPropertyChanged;
    }
}

@RayCheung0616 good to hear that :-)