The following is the approach I take when it comes to dealing with financial obligations - be it in taking up installment plans for a house, to upgrading a DVD rental service.(or my views of society in general)
If i was to base my decision to upgrade purely on the basis of my desire to watch more DVDs, i would upgrade. But if i was to base it purely on economic prudence, then, the following would apply.
Plan A - 2 dvd at home plan
subscription cost = 13.27
value of dvds received per month assuming each dvd is 2 pounds = 32 pounds
1 (pound of subscription cost) = 2.41(pounds of dvds received) (32 divided by 13.27)
Plan B - 3 dvd at home plan
subscription cost = 16.33
value of dvds received per month assuming each dvd is 2 pounds = 48 pounds
1 (pound of subscription cost) = 2.94 (of dvds received) (48 divided by 16.33)
loss incurred under Plan A for not upgrading to Plan B
loss = 0.53pence per pound (2.94 - 2.41 = 0.53)
loss per month - 7.03 pounds (0.53 x 13.27 = 7.03)
However, it might be more economically prudent to simply purchase the DVD if you would want to watch it a few times over a period of time. Renting it again and again would simply amount to a loss as the cost of purchase might be cheaper.
I'm still waiting for Dvds to be sold digitally at reasonable prices. If rental services can be this cheap, I really don't see why movies have to be sold at high prices, given that tons are already made via cinema goers. For the above prices, i can get between 16 to 24 Dvds a month, so, i sometimes wonder, why can't i just purchase those Dvds at the same or slightly higher cost - since I'm getting to keep the cheap DVD disc. Should it matter if I'm going to watch it once or a dozen times? That is irrelevant. It makes sense if we're going to eat a particular dish over and over again and thus pay for it over and over again. But it's not like the actors are going to be acting out the movie over and over again are they.
Sometimes I wonder if this sort of logic, on the part of the corporations, is that which is encouraging piracy. I also wonder if the 'war on piracy' is simultaneously a war on the people's ability to think rationally. So long as the people can get 'academic' and 'psychobabble' about things, corporations and governments cannot do as they please. But when people can get dumb enough to discount logical and philosophical reasoning by others amongst them as 'academic' and 'psychobabble' nonsense, that is when we know the elite have won. That is when what is right is determined by what is legal and not vice versa - as was the case the past thousands of years.