I am building a theater in my new house and I will need to run about a 25 foot HDMI cable from my projector to the dvd player. I was looking at places like Best Buy and an 8 feet cable is 100 bucks. I found 25 feet on monoprice for 38.00 while a 35 foot cable from bestbuy is 237.00 What is the difference?

The difference is the name stamped on the cable. It's all about marketing. Best Buy makes larger margins on their cables then on their A/V gear. So they don't want to offer inexpensive cables. Their profit margins would be reduced.