In the August 16, 1906 NYT is an interesting article describing how :

the New York National League Baseball Club [the NY Giants] brought suit against the Chicago National League Baseball club [the Chicago Cubs] for $3,500, alleging damages to that extent on account of the forfeited game of Aug. 7, when Umpire Johnstone was refused entrance to the Polo Grounds and gave the game to Chicago. Manager McGraw of New York wanted the game to be played with player from each of the teams as umpires, but the Chicago manager refused to agree to this, and the Ne York management now declares the club is damaged to the extent of the club is damaged to the extent of $3,500....The damages are said to lie in the loss of gate receipts.

Wait a second. New York bars an umpire, the game is forfeited to Chicago, and New York sues Chicago for damages?

Going back to the Aug. 8, 1906 NYT, here's what evidently happened.

Johnstone umpired a game between Chicago and New York on the previous day [Monday, Aug. 6] and ordered "Manager McGraw and Third Baseman Devlin out of the game. McGraw and Devlin received their notices of official suspension yesterday [Aug. 7] , and the action against Johnstone by the local club came immediately after...The New York Club officials refused to admit the umpire to the grounds, and he promptly declared the game forfeited to Chicago by the customary score of 9 to 0...Police Inspector Sweeney had requested the New York Club not to allow Umpire Johnstone to enter the grounds, as after threats against him it might precipitate a riot...For a time 8,000 spectators massed in front of the entrances and would not leave the grounds. Many of them were appeased with the announcement, however, that all reserved seat checks and rain checks would be good for any other game this season, and a subsequent announcement was made...that money would be refunded on all checks upon presentation at the office of the New York Club...

It seems that McGraw argued that if there was to be any forfeit it should have been the Cubs that forfeited to the Giants. This was because, according to the rules at the time, if an umpire was absent or unable to perform his duties, an agreed-upon individual chosen by the home team could substitute. McGraw claims he had a substitute ready to go at 5 minutes before 4pm (the start of the game) and the Cubs were the ones who walked away. Therefore, the Cubs should forfeit.

One wonders if this means that if the Cubs had forfeited the refunds would not have been offered by the Giants.

Back to the $3,500 claim, the price of admission to the Polo Grounds in 1906 was $0.50. This would imply an actual gate of 7,000 people versus the estimated 8,000 people (assuming all tickets were redeemed for a refund or another game). The attendance figures available in the Retrosheet.org game logs from 1906 indicate an average attendance at the Polo Grounds in 1906 of 7,200 with a standard deviation of 3,000 (albeit on only eight games).