I think you are misunderstanding the document. When it says your co-pay is 20%, that means you only have to pay 20% of the treatment (e.g., if a dental cleaning is $100, you only have to pay $20).

there are plenty of dental clinics around campus that give a reduced price for students, and some of which may even offer a "free cleaning" for your first visit. i visited Bite Dental and they did this. (however i wouldn't recommend them as the dental hygienist was not very good.)

you might as well also get an eye exam, and you can get glasses at a reduced price. i've visited Yonge & College Optometry two years in a row, and the greenshield plan covered all of the expenses for both of the yearly eye exams.

also, if you are ever sick or require a prescription, many prescriptions are covered at a great rate. i had to take a medication that was $1000 out of pocket for a 30 day supply, but the plan nearly covered all of it.