The stock honda exhaust isn't that restrictive on a stock 400ex, just remove your airbox lid and rejet the carb by playing with larger main jets. A prefilter or a modified/k&n lid is recomended as your tires will throw $hit right into your airbox.

 

Along with the Exhaust you could look at a Rev Kit
I did a White Brothers
This changes the ignition timing and allows for 1000 more engine rpm
I paid about $50
In the air box UNI makes Filtered Air Vents
They are simple to install and allow for more air into stock airbox without removing lid
Simple to install
You asked for cheep on budget
Can not have to many toys
Ray

2- 400Ex
1- 110 ATC
1- 185S ATC