I don't think MS Project is that overpriced, the professional version can cost as low as $700, and you get all the features that you need. Not to mention, of course, that it's not you who will pay for this software, but your company.
Now about the MS project free clone, there are many free project management software out there, but there is none that I would recommend. All of them are buggy, instable, and are not supported. Also, they all miss some important features that MS Project has.
I think you're stuck with Microsoft's products until OpenOffice decide to create an MS Project clone,