Well prepaid requires $15 per month (a little less with automatic payments). This gives you unlimited texting and you only pay for the minutes you use in addition to the $15 monthly. You actually buy a block of minutes, but they don't expire so long as you keep up with the monthly payment.
If you have very low usage you might want to check out something like SpeakOut. They work on the Rogers network, but that may be okay for Ontario. I don't think Koodo has a suitable plan aimed at people who have very low usage.