It really depends on the villager. I so far haven't sold any - I always have given my extras as trades. But one guy gave me a 1.5 million bell discount on some items I bought from him in trade for only 2 villager pictures (neither were any of the super popular ones).
On the other hand, someone traded me Ankha's picture (one of the most popular villagers) for an item that goes for about 500k bells on the Re-Tail forum here.
(Every other duplicate picture trade I've done can't be compared to prices due to how the trades were.)