I just ran into this same issue and managed to solve it. In my case, the model is being obtained via a RESTful $resource and the value for the amount is being provided as a string to the field, which in turn wipes out the value. In order to address this, I ended up doing the following in my controller:
$scope.cart = Cart.get(id: $routeParams.id, function(cart){
cart.quantity = parseFloat(cart.quantity, 10);
});
which turns the value into a float, prior to updating the view. One gotcha I ran into is that my first attempt was setting $scope.cart.quantity = parseFloat($scope.cart.quantity, 10) immediately after the get. This was not working since the value was overwritten when the async call to get completed.
$scope.cart = Cart.get(id: $routeParams.id);
$scope.cart.quantity = parseFloat($scope.cart.quantity, 10); // This won't work
Hope this helps.