Dude, it's simple economics.
People are willing to pay the most expensive tickets in all of baseball, and have bought EVERY ONE FOR 456+ games.
The supply is limited. The demand is tremendous and has resulted in a shortage (there aren't enough seats to satisfy demand). Therefore the cost of the good, the ticket, is very high.
By making the stadium larger, you are increasing the supply of tickets. You are looking to satisfy some of the extra demand for seats. If the supply increases, the price will decrease (maybe not right away, but eventually) to meet the market's equilibrium point.