There are many resources, at the career centre and elsewhere online, to prepare for case interviews. There is no need to pay for costly preparation support if you start well enough in advance.
The McGill Consulting Association Case book has many cases which you can use. The McGill Soutar Career Centre online portal has casebooks from other consulting feeder schools as well as interactive cases from consulting firm websites. The centre library also has many books about preparing for the case interview.
Use How to Conduct a Mock Case Interview to help you select good cases.
The Management Consulting Association also has many resources and activities that can help.
Case books:
McGill Consulting Association Case book
Haas 2006
HBS 2004
Kellog 2004
Michigan 2006
Stern 2007
Wharton 04/05
Wharton 05/06
Wharton 07/08
Chicago GSB 03/04
Case Interview Guides:
How to Conduct a Mock Case Interview
Ace the Consulting Interview
Vault Guide to the Case Interview (Create an account and search for the guide)
Ivy Case System
Consulting Math Practice Problems