I thought to add my own opinion on the subject. Admittedly, I am not a pilot, but am considering it in a few years. Nevertheless, I believe my opinion may be of interest.
All the talk above about "we will not stoop to their level" and "I personally have the honor to not take this job" or "we should band together, it isn't fair the company isn't paying us"; all of this is simply crap. Whether you like it or not, every working individual in this nation is a "whore" in the absolute sense. We all work for money. That's it, nothing more. Very few of us would ever go to work if we weren't paid - so we exchange our time and work for cash.
Furthermore, it is nowhere written down in this FREE MARKET ECONOMY that a company "shouldn't" do anything they like (as long as it is legal). If a company can get away by "selling" the right seat to those interested in turboprop time, then by all means, they should be allowed to do so. If a pilot is willing to pay the 18 grand, it is a win-win situation for both individuals.
To get my PPL, I have to pay to rent the plane and buy the CFI's time. In fact, to get many things in life (be it training, dinner, entertainment, rent, anything!), I have to pay for them. It doesn't matter that Gulfstream is receiving your service in flying their passengers around; you are paying for turboprop time. Conceptually, you couldn't care less if there are people in the back seat. Why should that matter that the company is also making money of paying passengers? Same number of turbo ME hours for you. You agreed to pay the 18 grand and you knew that you would be flying revenue passengers; it must be worth it TO YOU then. Some would say that isn't "fair". Concepts of "fairness" don't exist in business. Though many pilots are interested in being paid based on "fairness" and believe that their service is especially valuable, and therefore should be paid more - they are also full of crap.
The bottom line is that anyone's "service" is worth exactly as much as someone is willing to pay for it. I may think that I flip burgers especially well and should be paid $10 an hour since McDonald's Corporation makes millions in profit each year. But the bottom line is that Mickey D's will hire anyone they please - and there is some guy standing behind me that will work for less. That is the way a free market, capitalist economy such as ours works. Sometimes its harsh, but that's the reality of our nation. If you can't stand this reality, then you aren't ready to accept both the pros and cons of living in a free nation with the protected rights to pursue life and liberty. Perhaps you should consider a more pro-social government such as France and Denmark.
If you've got the money and think the Gulfstream deal is worth the turboprop time you receive, go for it. Otherwise, lay off. The reason that so many people turn away from this valid option is because others harp on it so much, and are resentful of someone spending the 18k they weren't willing to borrow themselves.
Don't kid yourself about fairness in this industry, or any other in the US. College is "pay for training", as zillions of other options to advance your skills in this nation- and there simply isn't anything wrong in doing so. You have to be realistic to suceed in any career, and one of those realizations is that you simply do what you have to do to advance your own interests. Choose your path wisely, and don't take anyone's advice without a grain of salt (mine included). Don't forget there isn't any "fairness" in this nation, so don't get bogged down in it. Just do what it takes.