Almost certain I have this question answered in a Pol Sci class in college, but that was 10 years ago and I can't seem to remember what it was.

Paying someone to have sex with you (prostituion) is illegal, in much if not most of the world. Paing two other people to have sex while you tape it (porn) is legal in most parts of the world. I suppose either because I't considered art (expression) or you aren't paying them to have sex with YOU.

But, what if someone got busted nailing a hooker and he was filming it.
Could he say that he was making a video starring himself and this paid "actress" to post on his website as artistic expression?

No I'm not asking because I plan on trying (more than happy with my wife), and not endorsing or championing legalized prostitution. Just seemed like an interesting conflict.

No that would be illegal (as long as we're speaking of countries where paying for a prostitute is illegal)
Just because he (assuming he!) films himself, or the act. Does not make it legal

I don't know for sure, but you may have to apply for some sort of buisness license to legally become a pornographer.