And I guarantee you it will be significantly higher than $100...certainly at Fenway Park.
As I suspected every field seat carries a ticket cost of $453.00 (not to mention $29.95 per ticket convenience charge from our good friends at TicketMaster).
Now I don't know the exact number of field seats, but an educated guess is about 6,000-6,500 (there's 16 sections, so I figured the 11 full sections to average conservatively about 400 seats each, 20 rows, 20 seats across, and 5 of the sections are smaller wedges fitting into the ballpark, though section C3 has at least 38 rows as I saw a C3 row 38 seat on the presale this morning for $453.00, its just probably only 10-12 seats wide.)
That'll be somewhere around 20% of capacity.
From what I've seen during presales, the $163.00 seats are the vast majority of the regular stadium seats with $63.00 and $102.00 being listed as obstructed view seats.
So for Fenway at least the avg. price will be a lot closer to $200 than to $100 (An educated guess is right in the $200 range).
The average may be less in some of the larger capacity football stadiums due to increased seating at lower price levels (Fenway held around 36,000 for Springsteen, football stadia will hold 60-80,000 and probably have the same amount of $400+ tix as Fenway so the average will drop in those buildings, but I'm guessing to about $150, not $100)