Purchasing a pass for the train or bus can be really expensive, especially with most monthly passes ringing in at the mid-hundred dollar range. It can be really hard to figure out each month if you're better off buying a pass or paying for each ride individually, which can save you a lot of money in the long run if you take a few minutes to think about it.
Below is a chart of the major transit agencies in the Toronto-area, and the amount of trips you need to take for a daily, weekly or monthly pass (depending on what's available) to be justifiable. The easiest way to think about it is to calculate how many trips you'll be taking to work or school, which stays the same generally for the whole month. Then, just find the agency and pass on the chart below and see which option is better for you.
I should note that the trip price for these calculations is based on the PRESTO adult fare. Unfortunately for people who are students/seniors or use cash or tickets to pay, this chart won't be of help to you.
(For example you need to take more than 41.76 trips in Oakville, so 42 or more, for the monthly pass to be economically worth it).
The math to do this is actually really simple; all you need to do is take the price of a single fare and divide the price of the period pass by it. That's how you calculate the amount of trips necessary to make the upfront cost "worth it," per se, and all that's left is to see how many trips you actually take in a month (which is really easy to track if you use PRESTO).