Obviously, there are more possibilities and therefore more interest on a longer railroad like D&S or C&TS. My local railroad, New Hope & Ivyland, charges $125 for it's annual steam photo freight with period freight cars, cab rides and multiple runbys at about 8 locations. The line is only 18 miles long, so photo locations are pretty standard each year. The past couple years, there's just barely been enough tickets sold to run the trip. I feel this is due to the limitations of the railroad- same loco, consist & photo locations each year. Still, it's 10 hours of steam & great fun.
Something more unique is naturally worth more. I travelled 2000 miles last year to ride #315, and all the way to Argentina for NG steam in 2007.