I have been, and its expensive. About $300-350/ticket. And they close the gates on thursday, with the main event happening on saturday night.
You will NOT be able to drive your vehicles around after you have made camp, and its a flat lakebed, so no offroading fun.
dont get me wrong- I love burning man, and would be going again this year (it would be my 5th year) if I didnt have a family issue to attend to- but I dont think it would be as fun for the vehicross crew as you may think.
But I can guarantee that you WILL have fun if you go.