I assume you're talking about example 3 from the citizensinformation site. The examples are using the 2006 guidelines, by the way, just to add to the confusion!
The limit for a GP visit card for a single person is currently €276, however there are allowances made for Rent/Mortgage, and travel to work.
When calculating entitlement, the HSE will always check if the person is eligible for a medical card first, and if they are not eligible, will then calculate entitlement to the GP Visit card.
In the example, the income is €320 and that's shown on the right column.
On the left column, the income guidelines for the medical card (2006 rates), allowance for Rent, and allowance for travel to work are added together to give €324. As this amount exceeds the applicant's income, the person qualifies for a medical card.
Given the figures you mentioned above for your cousin, he might actually qualify for a full Medical Card.
The limit for a medical card is €184, and if his full rent and travel expenses were allowed, that would add €145 to this limit - 184 + 145 = 329.
As this is greater than his income (280) he'd get the full card, not just the GP card.