409 and 509 are exactly the same. I have both. The difference was to whom and where they were marketed, AFAIK.
The difference between a 409 and a 414, is that the 409 is the perfect guitar and bass amp mic. There's something about its response that allows you to just throw it on and not have to use any EQ, and you get a great guitar tone.
A current fad is to use a SM57 with Royer 121. That's $1,200 worth of mic there! I prefer the sound of a single 409 to those two. And on a guitar amp, I prefer a 409 to any 414 that costs $500.
They were a $450 or so mic when they were brand new. They are still in high demand by the people who know them. The fact that they are no longer being made, and are still in demand dictates those prices. I own four of them. If I ever have to record amplified music, they go with me. They are my first choice for amps.