There will be a game of Base Ball between the Magnolia and Union Clubs, on the grounds of the latter, this afternoon, at 2 o'clock. As this game is not for the championship, no admission fee will be charged.
The Union Club have had their grounds enclosed by a high fence, and, hereafter, an admission fee of twenty-five cents will be charged for championship games.
-Missouri Republican, September 5, 1867
First, I think this may be the earliest reference to an enclosed St. Louis ballpark I've ever seen. Tobias has written that the enclosed Union Grounds were inaugurated on May 22, 1867, but this article implies that either the grounds weren't enclosed or admission wasn't charged until later in the season. And that is an important distinction. The general thinking among 19th century baseball historians is that wherever you find an enclosed ballpark there will also be a charging for admission (with an implication that the money would be used to compensate players). It's possible, but unlikely, that the Union Grounds were opened and no admission was charged until later in the season. It is more likely that the grounds were not enclosed until September of 1866 rather than May of that year. I just find it unlikely that you would have one without the other.
Second, it's interesting that the Union Club only charged for championship games. Obviously, a championship match against the Empires would draw a larger crowd than a friendly match against the Magnolias and people would be more inclined to pay to see a championship match. It should also be remembered that this was the first time in St. Louis history that people were being charged to see a baseball game and while the idea of charging for admission went back to the antebellum era, it was a novel concept in St. Louis. It was probably a smart idea on the part of the Union Club to phase in admission charges rather than immediately charging for all of their games.
Finally, we can now trace the history of the twenty-five cent ticket in St. Louis back to 1867. The twenty-five cent ticket had a big role in St. Louis baseball history and baseball history in general. It would be an important part of the St. Louis baseball experience into the 1890s and would help fuel the popularity of the game in the city. When ticket prices rose to fifty cents after the American Association and the National League merged in 1892, attendance suffered in St. Louis (although part of the problem was, of course, the very bad teams the Browns were putting on the field). Twenty-five cent tickets were the norm in St. Louis for almost thirty years and that put baseball within the financial reach of the common folks of the city.
Note: After writing all of this, I realized that the Nationals of Washington came to St. Louis in July of 1867 and played both the Empires and Unions at the Union Grounds. The grounds had to have been enclosed by then and admission was certainly charged. The Nationals weren't going to come to St. Louis unless they got paid and the only way they were going to get paid was if admission was charged. So I'd stick with Tobias' date of May 1867 for the inauguration of the enclosed Union Grounds.