I don't know how things go in other cities, but I would try looking at used media resale shops and pawn shops for DVDs and Blu-rays. Blu-rays that sell for $30 at retail a pawn shop may sell for $10-$15, same at used media stores. Many Pawn Shops in my area sell DVDs for $3-6 a piece, most are selling them for $3 and one even sells them for $2 a piece. These aren't copied DVDs, or DVDs without cases, and they even have decent selections.
Shop used, don't go to the places everyone else thinks to go (because that's where the higher prices are). I think 3 for $20 is way overpriced, considering how much money they've already made on the DVDs they could (*and often do) throw them away and still make money.
*Blockbuster actually throws away DVDs they don't sell in a short enough amount of time, and it's not just a few, it's thousands of DVDs each store throws away in just a few months. They aren't allow to sell them for less than the advertised price, and they aren't allowed to give them away, they are supposed to destroy them with the cases and all.