What are you looking for in your gun?
Softness of shot?
Lower tube parts. Mainly the springs and valve.
Smoothness and ease of pump action?
Springs in the Lower tube and Springs in the pump kit (if any).
Want something more consistent?
Lower tube, regulator and barrel.
Want something that will give you straighter shots?
Barrel and paint
Want something that will all around redefine the whole gun for under $100-$110?
Buy a maddmann spring kit and rocket valve. Then buy a .678 Lurker Eigenbarrel. Then buy a Bob Long 360 Regulator on here.
That should run you $25-35 for the valve and springs. $30-40 for the barrel, and $45-55 for the regulator.
Oh ****! I totally didn't notice!
Get yourself a decent clamping feedneck, my good man. Cocker threads. Function and Hybrid are inexpensive and extremely good. You can always go the Ego neck mod route as well...