Xamarin doesn't cost anything for Windows Phone. Xamarin Indie is reasonably unrestricted, but let's just go with Business for now:

    Order Summary
    Xamarin.Android Business    999    × 3    2997.00
    Xamarin.iOS Business        999    × 3    2997.00
                                            $5,994.00
    Multiple Licenses                  10%    –599.40
                                               Total:
                                            $5,394.60