#Household math - Which is the better way to pay for an online school when dealing with referral credits?

Question

I'm looking at joining an online school website that has over 22,000 courses in all sorts of different topics (I'm interested in some portion of them that relate directly to my interests). The cost for joining this website is $15/month but if I join for a year, I pay $99. To make things more complicated, I have 2 coupons for 1 month off each (one is the usual offer from the website and the other is as a referral coupon - I got one and the person who referred me also got one). What's the best way to join the website?

Answer

It's best to subscribe for a year, unless you are going to receive 2 or more referrals every month. 

Analysis

This is an interesting question because of the twists and turns in what's available in terms of options. The key is going to be to put everything on equal terms so that we're comparing "apples to apples".

Let's look first at what happens if we look at the options without regard to the coupons. We're comparing the regular monthly cost of $15 vs the monthly cost of $99 over 12 months, which is $8.25 per month. Clearly it's better to pay less per month! But... what if you don't use the site for all 12 months? What's the number of months that'd you have to use the site on the yearly plan to have it cheaper than paying $15/month?

We can find that by dividing the yearly cost of $99 by the monthly cost of $15. This gives 6.6, or in other words, it's better to pay by the month if you'll use the site for 6 or less months. For 7 or more months, it's better to pay the yearly amount.

Now let's look at the coupons. When paying by month, the coupons give 2 free months (and so for the cost of 1 month for $15, you get 3 months). When paying by year, you get 14 months for the cost of 12. What that works out to be is, when paying monthly, $15 for 3 months is $5 per month. When paying yearly, $99 for 14 months, that's $7.07 per month. And so there appears to be a better financial result to pay for 1 month and pay $15, use the site for 3 months, then change over to a yearly plan. However, that ignores a couple of factors, and so the best way to calculate this is to calculate the monthly cost over the course of the annual plan, then look at the associated monthly cost.

Using the yearly plan, you get 14 months for $99. Using the monthly plan first and then paying for the year, you get 15 months for $15 + $99 = $114. To compare the two, we divide the $114 by 15 (to get the per month rate), then multiply by 14 to get to the same number of months under the yearly plan:

And so it's best to pay for the year and get the 2 free months added on.

The one exception to this would be if there is an expectation of receiving 2 or more referral codes per month. If that's the case, it'd be better to stay with the monthly plan until the likelihood falls off of getting those referral codes. At 1 referral code or less per month, it's better to pay yearly.

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As always, feel free to ask a question!