Validating integer field with custom model binder in ASP.NET MVC

I ran into an issue that I thought was interesting.

Take an input form that has an input field for an integer value (cost, in my case). I do not want "$100" or "Twenty Dollars" to be sent to the database as it will likely complain. I want to add an error to the model and send it back to the user. Seemed simple ;).

I am using a custom model binder because I am using some inheritance in my domain model. I have multiple types of events, each type implementing IEvent - thus, I needed a custom model binder.

The issue I have is, when I am trying to bind the cost field, and the conversion from string to int fails, I am really not sure how best to handle this.

My Custom Model Binder public class EventModelBinder : IModelBinder { #region IModelBinder Members

    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        // Trivial code ... binding other fields...

        _event.ProjectedCost = GetA<int>(bindingContext, "ProjectedCost").GetValueOrDefault(-1);

        return _event;
    }

    #endregion

    // From Scott Hanselman's blog :)
    private Nullable<T> GetA<T>(ModelBindingContext bindingContext, string key) where T : struct
    {
        if (String.IsNullOrEmpty(key)) return null;
        ValueProviderResult valueResult;
        Nullable<T> ret;
        //Try it with the prefix...
        bindingContext.ValueProvider.TryGetValue(bindingContext.ModelName + "." + key, out valueResult);
        //Didn't work? Try without the prefix if needed...
        if (valueResult == null && bindingContext.FallbackToEmptyPrefix == true)
        {
            bindingContext.ValueProvider.TryGetValue(key, out valueResult);
        }
        if (valueResult == null)
        {
            return null;
        }

        try
        {
            ret = (Nullable<T>)valueResult.ConvertTo(typeof(T));
        }
        catch
        {
            return null;
        }

        return ret;
    }

This works really well - if Enums are null or not selected, I can validate against them and let the user know it is required. For the cost field, I get -1 if the conversion failed. But, when I go to validate the data on the server and go back to the UI, the Model.Event.ProjectedCost field is null

My EventService

// ...
protected bool ValidateEvent(IEvent eventToValidate)
    {            
        if (eventToValidate.ProjectedCost < 0)
            _validationDictionary.AddError("ProjectedCost", "Incorrect format");

        return _validationDictionary.IsValid;
    }

    public bool SaveEvent(IEvent _event)
    {
        if (!ValidateEvent(_event))
            return false;
        try
        {
            _repository.SaveEvent(_event);
            return true;
        }
        catch
        {
            return false;
        }

My Edit.aspx View

<h2>Edit</h2>

<%= Html.ValidationSummary("Edit was unsuccessful. Please correct the errors and try again.") %>

<% using (Html.BeginForm()) {%>

    <fieldset>
        <legend>Fields</legend>
        <p>
            <label for="ProjectedCost">ProjectedCost:</label>
            <%= Html.TextBox("ProjectedCost", Model.Event.ProjectedCost) %>
            <%= Html.ValidationMessage("ProjectedCost", "*") %>
        </p>

        <% Html.RenderPartial(String.Format("~/Views/Shared/{0}Form.ascx", Model.Event.Type), Model.Event, ViewData); %>

        <p>
            <input type="submit" value="Save" />
        </p>
    </fieldset>

<% } %>

<div>
    <%=Html.ActionLink("Back to List", "Index") %>
</div>

In my view, Model.Event.ProjectedCost is null if it doesn't validate, giving me this (Line 54 is the culprit):

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 52:             <p>
Line 53:                 <label for="ProjectedCost">ProjectedCost:</label>
Line 54:                 <%= Html.TextBox("ProjectedCost", (Model.Event.ProjectedCost == null ? 0 : Model.Event.ProjectedCost)) %>
Line 55:                 <%= Html.ValidationMessage("ProjectedCost", "*") %>
Line 56:             </p>

What I'd kind of like to do is send the value the user entered back to the user, but my custom model binder and/or validation logic seems to be setting something to null?

I realize this may not be the easiest question to read, so let me know if I can clarify in any way!