The double fairlie's price is only 1000 to buy and 0.10 per km while at the same time it is much more powerful than the only contemporary narrow gauge engine "George England" which costs 17000/0.89. Also the later "Single Fairlie" has a normal price. I consider buying the Double Fairlie an exploit for that reason an refrain from doing so.
This sounds like someone just forgot to change the default costs when the dat-file was made.