The cost is what the market will bear. I personally think it's gotten too high, but DirecTV will price it at a level that attracts enough subscribers to recoup their (major) costs for the package plus a profit.
Other providers had the opportunity to bid on the package. DirecTV won. The NFL does not want it to be available to all providers, as that would cause additional friction with the networks and their affiliates.
+1 If Directv lost exclusivity with the Sunday Ticket package, the NFL would have the package priced close to what Directv charges as do the other sports packages. There would be some migration back to cable but I don't think that much. We will see when the package is back up for renewal