Say rental is $8/month and approved docsis 3.1 modem is $180 to buy outright. Buying doesn't really make sense unless you will be sticking with the same ISP for at least two years. (And in two years, you might take a promo deal and sign up with another ISP; or move to a different apartment building which doesn't allow your current ISP; or your roommate or SO might decide to take care of connectivity; or technology will advance and you would want a different modem anyway.)