I went to a Cardinals game at Wrigley this year, in one of the worst Cub seasons of all time (Cubs still won). Tickets for the game were not all that hard to come by, but overpriced on Stubhub, Craigslist, etc. If you know now that you are going, and can get the tickets via the Cubs web site, do that. They probably are not going to get any cheaper because people will pay to see the Cardinals and the St. Louis fans travel well. The other thing is that the Cubs tiered their games and charged more for the games they knew would be high demand, and the Cardinals ticket was in the highest tier. So a seat that might cost you $20 on a Tuesday against the Marlins will cost you $75 on a Saturday against the Cardinals.