KO numeric extension.

bindings.js

ko.bindingHandlers.number = {
    update: function (element, valueAccessor, allBindings, viewModel, bindingContext) {
        var value = ko.unwrap(valueAccessor());
        valueAccessor = function () {
            return value.toString().replace('.', ',');
        };
        ko.bindingHandlers.text.update(element, valueAccessor, allBindings, viewModel, bindingContext);
    }
};

bindings_test.html

<span data-bind="number: 1.5"></span>

extensions.js

ko.extenders['numeric'] = function (target) {
    var result = ko.computed({
        read: target,
        write: function (newValue) {
            if (typeof newValue === 'string') {
                newValue = newValue.replace(',', '.');

                newValue = parseFloat(newValue);
                if (!isNaN(newValue)) {
                    target(newValue);
                }
            }
            else if (typeof newValue === 'number') {
                if (!isNaN(newValue)) {
                    target(newValue);
                }
            }
        }
    });

    return result;
};

extensions_test.js

var observable = ko.observable(0).extend({numeric: true});

observable('1,5');
observable(); // 1.5