Unless you are in NY or LA, that price is borderline rape. However, to answer your questions:
-Why are personal trainers so expensive?
That depends on a lot. The price that you quoted above is definitely on the high-end. However, there are several things that contribute to the trainers hourly rate: If the trainer works for a "Big Box" gym, it is very possible the club sets his price, these tend to be on the high side. Freelance trainers have a lot to consider when setting an hourly rate. If the trainer is in high demand, they tend to have a high rate. This is actually one of those situations that you get what you pay for (most of the time). If the trainer is in a large market (like LA or NY) they don't depend on consistent clientele, and can set the price high. If they size you up as a "high maintenance" client, they will typically charge you more. Its not fair, but its the business.
- Moreover, are they effective?
In short, yes. HOWEVER, you need to size them up first before committing. Ask around your gym. Find out what the general attitude is about that trainer. If people say "Yeah, he/she is good, I lost xlbs in x weeks" STAY AWAY. If clients or former clients WARN you about how hard their program is, sign yourself up.
I am an elite athlete and I have a trainer. I am very capable of training on my own, but she puts me at the level that seems almost unreachable on my own. She commands a heavy price, but she knows her sh1t. Not only is she knowledgeable about human anatomy and biology, but she is a motivator. On a side note, she is a former American Gladiator. Funny, now that I actually think about it, I pay someone an absurd ammount of money to make me hurt so bad, I can't even sit without crying like a 3 year old... And I'm not even into BDSM...