You have to remember that duels are on the reserved list, have not been printed since Revised (OOP in 1995) and wont ever be printed again.
I have seen them rise in price over the years and I think it unlikely they will fall.
They are also staples in the Legacy and Vintage tournaments.
There's a lot of fakes around so do your homework if you buy. Revised are the cheapest, and Alpha the most expensive.