The retail price for new games in my country(Australia) are $100. This price is around $80 US.
I bought the 30 games from JB Hi-Fi a discount chain store. I traded in 30 games which cost on average $15 each. I traded in 30 copies of three different games. 10 copies of the same game. 10 separate trades to get the 10 free copies of Mass effect 2. Each set of three would have three different games. The game store has exclusion lists. I buy the cheapest and crappest games not on exclusion list and trade them back in and get the free game. Total cost of the 30 games was $450
All up I made $485 in profit. Trade store credit when I traded in the 10 copies of Mass effect 2 in one trade at one EB Game store. I got $935 in store credits.