I have a 04 400ex and when I hit the gas sometimes, blueish smoke comes out the pipe. Does that mean I need a rebuild and how much would it cost?!?!? Need help please

 

Sounds like you are starting to burn some oil. A rebuild or top end work is in your future. If you just replace the piston and give it a bore you are looking at 200-225. But while you have it apart it would be wise to replace the cam chain and have the valves checked out.
What is your budget and how much ***** do you want your ex to have? The ex is easy to work on, real easy. Before you even think about doing the work read the manual. If you think you can do the work yourself give it a try. All you need is a metric socket set and a torque wrench.