Edit in JSFiddle

Result
HTML
JavaScript

<!DOCTYPE html>
<html>
    <head>
    </head>
    <body class='ui-widget'>
        <header class='ui-widget-header'>
            <h1>Give me all of your money</h1>
        </header>
        <div class='ui-widget-content'>
            <p>
                <label>How much in Cash?</label><input data-bind="value:Cash.formatted,css:{negative:Cash.isNegative}" />
            </p>
            <p>
                <label>How much in Checks?</label><input data-bind="value:Check.formatted,css:{negative:Check.isNegative}" />
            </p>    
            <p>
                <label>Total:</label><span data-bind="text:Total.formatted,css:{negative:Total.isNegative}" />
            </p>   
            <p>
                <button data-bind="click:showJSON">Show View Model JSON</button></p>
        </div>
    </body>
</html>

(function(){
    var format = function(value) {
        toks = value.toFixed(2).replace('-', '').split('.');
        var display = '$' + $.map(toks[0].split('').reverse(), function(elm, i) {
            return [(i % 3 === 0 && i > 0 ? ',' : ''), elm];
        }).reverse().join('') + '.' + toks[1];

        return value < 0 ? '-' + display : display;
    };

ko.subscribable.fn.money = function() {
    var target = this;
    
    var writeTarget = function(value) {
        var stripped=value
            .replace(/[^0-9.-]/g, '');
        
        target(parseFloat(stripped));
    };
    
    var result = ko.computed({
        read: function() {
            return target();
        },
        write: writeTarget
    });

    result.formatted = ko.computed({
        read: function() {
            return format(target());
        },
        write: writeTarget
    });
    
    result.isNegative = ko.computed(function(){
        return target()<0;
    });

    return result;
};
})();

//Wire it up
$(function() {
    var viewModel = {
        Cash: ko.observable(-1234.56).money(),
        Check: ko.observable(2000).money(),
        showJSON: function() {
            alert(ko.toJSON(viewModel));
        }
    };
    

    viewModel.Total = ko.computed(function() {
        return this.Cash() + this.Check();
    }, viewModel).money();
    ko.applyBindings(viewModel);
});