If you purchase the iPhone outright, you can get the BYOD discount.
Premium phones with high levels of subsidy come at a higher monthly price. The carrier has to recoup the $500 subsidy they grant you somehow.
$80 per month with only 500 megabytes of data? That is a rip off. It basically turns your expensive iphone into a phone capable ipod touch as you would be permanently stuck on wifi if you are to do any internet surfing whatsoever (even using onavo extend or chrome browsers built in data compression)...
You're right, $80 for just 500MB would be a rip off but there are plenty of other included features, not to mention the subsidy for your phone ($20/month). The plan also comes with unlimited calling in Canada and texting, something MANY people will happily pay a premium for to avoid overage charges.
Also, this isn't your only option, you can pay $5 more and get 1GB or $10 more and get 2GB (plenty for an iPhone).