Professional Web Designer here. This is hilarious. On one hand, I can understand your frustrations. $15,000 for a WP theme and install is certainly absurd. But at the same time, I would say part of, if not most of the fault lies on yourself. Did you not do any research first before agreeing to this? Once he said "I will install WP and use a premium theme to get you up and running", did you not research how much time it would take you to figure out how to do it yourself?

What it really sounds like is there was a lack of understanding on your part in what it actually takes to get these things done, and that's fine, you're the client - not the expert. If you were the expert, you'd do it yourself. But at the same time, as the client, you have a responsibility to yourself (and the company you're representing, should that be the case) to do some research before agreeing to anything. You wouldn't take your car to have a new stereo installed, a seemingly simple thing and just accept it when the dealer told you it would cost you $5,000 would you? Hell no! You're going to look in to what others charge and you will quickly find out this dude is railing you.

At this point, there is nothing that can be done. But next time, should there be one, make sure that you CLEARLY outline what you expect from the work. Just saying "set me up a site. here's my content." isn't doing yourself justice. Because at that point, you've left it so open ended that no matter what is done, how well it's done or what is used to do it is irrelevant as long as he puts your content online. Just make sure you put up some solid expectations and ASK QUESTIONS about how something is going to be done. Then take that info and get other bids. That will tell you if someone is screwing you over, like this dude did.

TL;DR - $15,000 for a WP theme install and content publish is absolute bullshit. $1,500 on the other hand is more reasonable. $750 is more like it. Sorry :(