Could it be that the porn scene is only 10-20 minutes long, and the lady are offering 1-2 hours?
Just a guess.

Porn stars are...stars.  There are many guys willing to give their left nut (or shell out big bucks) to see ladies they have rubbed out a few with watching them in movies.  

Me, too.  I once did that...amazing experience...

Because they can....

I'm not in the business so I'm not privy to rates across the industry.  But SF fetish studio Kink.com posts their per scene rates on their Web site:

http://beta.kink.com/k/model_call.jsp

Rates vary from $500 per scene (solo dildo play) to $1300 (double penetration) per scene depending on the type of scene.  According to their rates page scenes generally take 3 - 4 hours to shoot.

As for the differences between paid for a scene verses a private encounter understand that in most cases the scene will:

+ likely involve professionals that the lady already knows and/or has worked with before

+ often give the lady an opportunity to to say yes/no to choice of partners she will be working with ahead of time

+ involve an environment (location, directors, etc) that she is already comfortable with

+ have partners that have recent STD tests

+ allow her to take breaks or step out of scene/character as necessary

In a private setting:

+ the lady is probably meeting someone that she doesn't know anything about

+ there is probably not an exchange of tests

+ they lady has to be "on" for the full time of the session.

+ the environment may not be one in which she has a lot of control (e.g. your hotel room)

I could go on but I think you get the point.  There are a lot of differences so the two rates are not necessarily comparable.

Rob

Porn is legal, escorting is not so there's the risk of getting busted.  Also, when shooting a porn scene, she knows or knows of her partner and she's surrounded by a crew of professionals and people know where she is.  When she's escorting she's by herself and opening her hotel door to a complete stranger who may be a whacko or worse, so the risk of personal safety is much higher, hence the higher costs.

A different take on it

If it were as simple as risk differential that would not begin to account for the difference in price.  By way of example, every escort has that risk, yet they don't charge pornstar rates.

And if I were a girl, I would feel safer with a 50-year-old guy driving a mini-van from his kid's soccer practice than a porn guy who has had his dick in a thousand girls' mouths, pussies, and asses.  

If it were the risk difference, then the price for a regular guy should PLUMMET for a second visit, because then she knows you are not LE and you are safe, and you would even travel to her.  But the way they screen, they pretty much know that from the get-go.

Girls wannabe stars.  And THEY are paying (with cheap booking fees for porn shoots) for the lottery-style CHANCE to become rich and famous.  Those 500-800 rates are what the porn market will sustain, so that is all they get.  

IT is the agencies who elevate the price.  They provide a service, connecting guys who don't wanna do research with girls who don't wanna do their own marketing.  

After I see a girl I like I tell her if she is ever just sitting around watching DR. Phil and bored to call my hobby phone and I will come right over (at a price point that if I mentioned it on this board certain wise asses -- or guys who refuse to believe they overpay -- would ridicule me for, so I will PM it to guys I recognize if you care).   Then every now and then I will email the girl with a follow-up reminder that I am still out there.  If they ever have an opening and wanna make a quick $$ or $$$ don't for get my hobby number.  You would be amazed at how often I get that phone call. . . .   AMAZED.  Any you have nothing to feel guilty about, because even at the "rock bottom" rate, she is still making more per hour than you or I ever will dream of....