I may not be in the best position to express an opinion on this subject, as I have yet to visit an SP (plan to remedy this very soon), but I'll go ahead anyway.
I don't expect that I could ever justify spending $1000/hr for an SP but that doesn't mean others can't. We all spend our money based on our personal preceptions of "value". I own an economy car because I don't preceive the value of a high end sports car but for those who get pleasure from driving, a high end car is a reasonable expense.
I am generally very careful how I spend money, trying to avoid "impluse buying" as much as possible. Having said this I don't have a problem with an SP charging a few hunderd dollars an hour. If you compare this with the rates charged by consultants in many fields for example it is not outrageous. Some might argue that there is a difference between the service that a business consultant charging $300/hr provides and the service provided by an SP but this difference in my mind is only in the details. Ideally, both have special attributes that allow them to provide the required services. Understanding the customers requirements is important for both the consultant and the SP, if they don't their customer will not be satisfied and business will suffer. The main difference is that the business consultant is usually billing a corporation while the SP is billing a consumer.
I don't think being a GOOD SP is easy, in addition to the issues a business consultant has to deal with, an SP has to understand the risks associated with the various services they are providing and how to best minimize those risks. I expect that at times some customers make requests for an SP to provide extra service which imply greater risk, requiring the SP to decide if they are comfortable with the request or in most situations, I expect, being able to say "no" and potentially disappoint the customer. Unlike a business consultant, there is the possiblity of emotional issues arising at least in a few cases although it appears people try to prevent this from happening.
Independent SP's also need a range of business skills. I don't expect that you just decide to become an SP one day and your calander is filled the next. While some simply put an advertisment in the classified section, I expect this has limited success at the higher price range. From what I've seen many have tasteful web sites and allow you to correspond with them via email in advance of meeting them. As mentioned in an earlier post, SP's also have additional expenses such as travel costs, wardrobe (I'm guessing here), and rent in the case of those who provide incall services. When you are paying $300/hr to meet with an SP, that has to cover the time they have spent working to get you to meet with them.
Remember, as much as you might like to imagine otherwise, this is a business transaction which you have decided to participate in. Decide what you are really looking for and then determine what it will cost. Since the service isn't a necessity of life, you can always decide not to proceed if you think the cost isn't justified.
The one aspect I haven't addressed thus far is price negotiation. I expect that most people are used to some price negotiation in most business transactions and so the "non-negotiable" policy of most SP's may be an issue for some. I know that my rates are negotiable but that they vary primarily based on expectations of a continued, hopefully long term business relationship or if the contract is for an extended length of time. I'm not entirely how this would apply for an SP except in terms of long term repeat visits.
Finally, my attitude is that if I do enjoy meeting with an SP, then I will view it as a "luxury" activity, a special treat that I can provide for myself on an occassional basis. Thus, spending $500-600 for a few hours isn't outrageous.