Why is it so much easier(cheaper) to get a strong automatic transmission than it is a manual trans? Since the autos are so much more complex I'd think a manual would be much cheaper, but richmonds T-56 is about $3,000 and can handle what 450 HP? While Probuilts autos are half that cost and handle far more power.
Is it the planetary gear train that makes it able to take the power?

 

You guessed it. Not to mention less gears in most cases. Just look at the simplicity of the M-20, M-21, M-22s. Those are 4 speeds and cheap cheap cheap. The 5 speeds are more and the 6 speed is only the same price as the 5 speeds because GM used them as OEM for a while now.
Also an Auto has a TC which is a buffer on the inirtia produced by the engine. A stick on the other hand is brutal when you pop out the clutch and hit into the next gear (broken u-joints, broken tranny, broken mounts, broken diff when using big tires).
Also an Auto is more popular in production than any 5 speed, look at the mustang guys and you'll notice that the 5 speed isn't very expensive for obvious produciton number differences and popularity.