Revit LT has been recently released, this will set you back a little more than 1000$. Revit is a pretty difficult software to learn that I have trained professionals on for more than five years, it usually takes a few weeks for trained CAD professionals to get up to speed. Revit is a Building Information Modeling software that uses a parametric relational database to track changes between components, its quite sophisticated.
For renovation, there is an web application from Autodesk, that will allow you to make simple changes - http://www.homestyler.com/ - I would suggest hiring an architect or designer, of course. Sketchup is a good basic 3D Modeler that is free and can create scale drawings. Easy to build a static model but more difficult to accomodate changes.