Its not trans-fats, its artificial trans-fats. Really, though, Im not a business man and I have no idea how much it would be to use... "real" trans-fats.
Do I think foods taste better with trans-fats? I have no idea. I have no care for my eating habits so I don't know whether or not a food has a trans-fat.
But, I know you want more than that. So, I googled "Why does McDonalds use artificial trans-fats"
Well, I didnt get an answer but I did get something else. A businessweek.com news report saying that immediately when the law was passed McDonalds complied and took out their trans-fats.
As to your notion about spending a little bit of money. Its not about the trans-fat, according to this article which states....
"McDonald's says there aren't yet good substitutes for trans fat and that switching may compromise the taste of its food. Using trans-fat frying oils also allows fast-food chains to trim costs, since the same oil can be used for weeks."
As Well As
"...it [McDonalds] has said in the past that the company's priority is to meet taste and quality expectations of customers. It originally pledged to get rid of trans fat in its restaurants by 2003. But so far, it says, it hasn't been able to find an adequate substitute. "
Note that I do not know if they are being honest. I mean, there are some companies that SAY (once again, its their word we are relying on) that they removed trans-fats without compromising tastes. But really, how would we know?
I hope that answers all your questions.