I wrote a long a comment theorising why $100/year is a great starting point and then discovered you can toggle between "yearly" and "monthly". So they do offer $10/month -- it's just hidden behind a toggle.
Only viewing total cost misses the point. For some people, it may be more reasonable for them to want to pay a fraction of the cost monthly vs. one big sum every 12 months. $10/month is more expensive, in total, over a year. However, $100 is more expensive in the short-term.
Also, paying monthly is only more expensive if the person paying decides to stay that long. Monthly payments allow flexibility to cancel rather than committing to a year.
For it to make sense to you, imagine somebody buying a pizza:
- How much is the pizza?
- It's 10 dollars.
- I don't think it's worth 10 dollars!
- Uhm, okay then don't buy it.
- But I really need that pizza, it's really important that I get it!
- Then pay 10 dollars?
- That's too expensive, it's worth nowhere near that!
- If it's not worth 10 dollars why is it important to you?
- It's extremely important that I get that pizza and I really want it, but also you have to understand that it's not worth 10 dollars, so give it to me for free now!
...ad infinitum.
If it still doesn't make sense, then it will never make sense.
No comment on whether either is reasonable but responding to “it costs too much” with a more expensive option is an interesting choice.