Is there an average price they will pay for a dvd?

Depends on the number of copies they have in stock if they have a lot of copies you won't get a lot. If they don't have a lot of copies you will get more. You won't know until you take it up there. Just remember they sell DVDs for 1 dollar to about 5 dollars average. So either way don't expect anymore than a few dollars depending on the title

Depends on demand, quality of DVD and whether blue ray or regular

Depends on the rarity and condition. And yeah, expect a low buy

Price they pay depends on condition and demand.